An alarm that does not go off is a uniquely costly software failure. And unlike most phone problems, this one usually has no error message at all: the phone did exactly what it was configured to do, and what it was configured to do was stay silent.
The short answer
The three usual causes are a focus or do not disturb mode that is not allowing alarms through, battery optimisation suspending the clock app, and the alarm volume being a separate slider from the ringer that has been turned down. Check all three, then set a test alarm two minutes ahead to confirm it actually sounds.
Alarm volume is its own thing
This catches people constantly. Most phones have several independent volume channels: ringer, media, notifications and alarm. Turning the phone down using the side buttons usually adjusts whichever channel is currently active, which is often media. It is entirely possible to have the phone on full ringer volume and the alarm channel near silent.
Find the alarm volume specifically, in either the sound settings or the clock app’s own settings, and set it deliberately. Some phones also have a gradually increasing alarm option that starts almost inaudibly, which combined with a low ceiling means you sleep through the first minute.
Set the alarm volume explicitly
In sound settings, find the alarm slider, separate from ringer and media, and set it high. Also check the clock app’s own settings for a gradual increase option and decide whether you want it.
Allow alarms through focus and do not disturb
Open your focus or do not disturb settings and confirm alarms are on the allowed list. Sleep-focused modes in particular can be configured to silence everything, and the default is not always what you would expect.
Exclude the clock app from battery optimisation
On Android, set the clock app to unrestricted battery usage. An aggressively optimised phone can suspend the clock app overnight, and the alarm then fires late or not at all. This is the most common cause of an alarm that works some days and not others.
Turn off any third-party battery saver or cleaner app
These frequently kill background services regardless of exclusions you have set. If you have one installed, it is a prime suspect.
Check the alarm is actually enabled and set for the right days
It sounds obvious. Check whether it is set for weekdays only, whether the toggle is on, and whether you are looking at an alarm you dismissed rather than snoozed.
Check the alarm sound file still exists
If you set a custom tone from a music file and that file has since been deleted, moved or removed from a streaming download, the alarm may fire silently. Switch to a built-in tone as a test.
Check Bluetooth is not routing the sound away
If the phone is connected to earbuds in a drawer or a speaker in another room, the alarm may play there. Some phones route alarms to the phone speaker regardless, and some do not.
Test it
Set an alarm for two minutes from now, lock the phone, put it down and wait. This is the only way to know. Do it again after any settings change.
After any system update, test your alarm. Updates commonly reset focus mode configurations and battery optimisation exclusions, and an alarm that has worked for a year can quietly stop after an update you barely noticed.
The battery optimisation problem
Modern phones aggressively suspend apps that have not been used recently, to save power. The clock app is a natural target: you open it rarely, and it spends most of its time doing nothing. When the system suspends it, the scheduled alarm may not fire at its exact time, or at all.
Excluding the clock app from optimisation is the fix, and it costs almost nothing in battery terms because the clock genuinely does nothing most of the time. On phones from manufacturers with particularly aggressive power management, this is not optional; it is the difference between an alarm that works and one that does not.
If an alarm genuinely matters, do not rely on a single phone. A cheap standalone alarm clock costs very little and has none of these failure modes. For anything where missing it has real consequences, redundancy is the sensible answer.
Alarm apps versus the built-in clock
Third-party alarm apps with puzzles, shake-to-dismiss and similar features are more vulnerable to all of the above, because they are ordinary apps with no special system privileges. The built-in clock usually has exemptions that third-party apps do not. If your fancy alarm app keeps failing, the built-in clock is more reliable even if it is less clever.
Why did my alarm not go off even though it was set?
Most often battery optimisation suspended the clock app, or a focus mode silenced it. Both fail silently, which is why the alarm appears to have simply not happened.
Does do not disturb silence alarms?
It depends on how it is configured. Alarms are usually allowed by default, but sleep modes and custom focus profiles can be set to block them, and people rarely check.
Why is my alarm quiet?
Alarm volume is a separate channel from ringer and media. Turning the phone down with the side buttons usually adjusts something else. Set the alarm slider explicitly.
Will my alarm work if the phone is on silent?
On most phones, yes, because alarms bypass the silent switch by design. It is worth confirming on your specific phone rather than assuming, with a two-minute test.
Should I use a third-party alarm app?
The built-in clock is more reliable, because it has system privileges that ordinary apps do not. If you use a third-party app, be extra careful with battery optimisation settings.
The bottom line
Set the alarm volume explicitly, allow alarms through your focus modes, and exclude the clock app from battery optimisation. Then set a test alarm two minutes ahead and confirm it sounds, because that is the only check that proves anything. Re-test after every system update, since updates reset exactly these settings.
