The short answer
Battery optimisation is the cause in the overwhelming majority of cases. Set the affected apps to unrestricted battery use, check any manufacturer battery manager separately, and make sure the app is excluded from adaptive battery. On iPhone, check Focus modes and notification summaries, which delay by design.
Push notifications work by the phone holding a single open connection to a push service, which wakes the relevant app when a message arrives. Anything that suspends that connection or prevents the app waking delays the notification, sometimes by hours, and the phone gives no indication that it is doing so.
Android: the layers to check
Android has several independent systems that can delay notifications, and manufacturers add their own on top. Work through all of them, because fixing one while another is active changes nothing.
Set the app to unrestricted battery use
Settings, Apps, choose the app, Battery, and select Unrestricted. This is the single most effective change.
Turn off adaptive battery for that app
Settings, Battery, Adaptive Battery. It learns which apps you use rarely and restricts them, which is exactly wrong for a messaging app you check rather than open.
Check the manufacturer's own battery manager
Samsung, Xiaomi, Huawei, Oppo and OnePlus all add aggressive app management with names like Sleeping Apps, Battery Saver, App Launch and Protected Apps. Find it and add the app to the allowed list.
Disable Remove permissions if app is unused
Settings, Apps, the app, and turn off the option to remove permissions and free space if unused. This can revoke notification permission on an app you rarely open.
Check the app's notification categories
Modern Android splits notifications into channels, each with its own importance. A channel set to silent or minimised will not alert you even though notifications are technically enabled.
Check Do Not Disturb and its schedule
Including any automatic rules that trigger during work hours or at night and quietly suppress everything.
Task killers and memory cleaner apps are actively harmful here. Closing an app from the recents list, or having a cleaner do it repeatedly, prevents it receiving push notifications on many Android versions. If you swipe apps away out of habit, that alone explains late messages.
iPhone: the settings that delay by design
- Scheduled Summary collects non-urgent notifications and delivers them at set times. Apps added to it will always appear late. Settings, Notifications, Scheduled Summary.
- Focus modes filter by app and by person, and a Focus left on, or one with an automation you forgot, silences things all day. Check the list of allowed apps in each mode.
- Low Power Mode reduces background activity, which can delay fetching for some apps.
- Background App Refresh matters for apps that fetch rather than receive true push. Enable it for the apps that matter.
- Notification delivery settings per app, including whether the app is allowed Time Sensitive notifications, which bypass Focus modes.
The network side
Push notifications need a maintained connection to the push service. Things that break it:
- A VPN that drops or reconnects frequently, interrupting the push connection each time.
- A restrictive network, such as a corporate or guest Wi-Fi blocking the ports push services use. The symptom is notifications arriving instantly on mobile data and late on that network.
- Weak Wi-Fi where the phone keeps the connection nominally alive but loses packets.
- A phone switching between Wi-Fi and mobile repeatedly at the edge of coverage, re-establishing the connection each time.
- Private DNS pointed at a blocking resolver that filters a domain the push service needs.
A quick test: turn Wi-Fi off and see whether notifications arrive promptly on mobile data. If they do, the network is the problem rather than the phone.
Ask someone to message you and time it. Testing with a real message is the only way to know whether a change worked, and it avoids the common situation of changing six settings at once and never learning which mattered.
App-specific causes
- Email apps using fetch rather than push check on a schedule by design. Set the account to push where supported, or shorten the fetch interval.
- Apps signed out in the background stop receiving notifications while appearing normal until opened.
- Multiple devices logged into the same account can route notifications to whichever was most recently active, which is by design in several messaging apps.
- The app’s own notification settings, separate from the system’s, are frequently the answer and frequently overlooked.
When nothing works
Reinstall the app, which re-registers it with the push service cleanly. If several apps are affected on the same phone, the problem is system-level: check for a system update, and consider resetting network settings, which clears a corrupted connection state without affecting your data.
If a phone from a particular manufacturer has a reputation for this, and several do, searching the model name with the phrase delayed notifications will usually find a community-documented list of the specific settings that manufacturer uses to restrict apps.
Why do notifications arrive when I unlock my phone?
Because the app was prevented from running in the background and only receives the pending message when the system wakes it. That is the signature of battery optimisation.
Does closing apps save battery?
No, and it delays notifications. Android and iOS manage background apps far better than manual closing does, and an app closed from recents may be prevented from receiving push messages entirely.
Why are my notifications fine on mobile data but late on Wi-Fi?
The Wi-Fi network is interfering with the push connection, either through blocked ports, a filtering DNS or an unstable link. Test on a different network to confirm.
What is notification summary on iPhone?
A feature that batches non-urgent notifications and delivers them at scheduled times. Apps included in it will always appear late, which is exactly what it is designed to do.
Will a factory reset fix delayed notifications?
Rarely worth it. The cause is almost always a specific battery or notification setting that returns after you set the phone up the same way again. Work through the settings instead.
The short version
Set the app to unrestricted battery use, exclude it from adaptive battery, and check the manufacturer’s own battery manager, which is the layer most people miss. Stop swiping apps away from recents. On iPhone, check Scheduled Summary and Focus modes. Then test with a real message on both Wi-Fi and mobile data to see whether the network is involved.
