The common reasons why an iPhone alarm may not be going off include:

• Attention Aware Features turning down the alarm volume if you are looking at the phone (iPhone X and later).
• Volume settings being too low or the mute switch on the side being on.
• Do Not Disturb or Sleep Focus modes silencing the alarm.
• Incorrect alarm settings such as wrong time or no sound selected.
• A software glitch or outdated iOS version.
• Connection to external devices like Bluetooth headphones, causing the alarm sound to play elsewhere.

To fix the problem, try these steps:

1. Turn off Attention Aware Features in Settings > Face ID & Attention.
2. Check and increase volume in Settings > Sounds & Haptics.
3. Ensure the mute switch is off.
4. Turn off Do Not Disturb or Sleep Focus modes.
5. Verify alarm time and sound in the Clock app.
6. Restart your iPhone.
7. Update to the latest iOS.
8. Disconnect from any external audio devices.
9. Reset all settings if needed, via Settings > General > Transfer or Reset iPhone > Reset All Settings.

If the issue persists, it may be a hardware problem and visiting an Apple repair store is advised.