My alarm just doesn't go off sometimes

"The volumes set to zero!" "You have the wrong time set!" "You're just sleeping through it!"

No, wrong, shut up. I checked that.

Sometimes I need to wake up at a certain time so I'll turn on an alarm for the morning. I will select 9:00 AM, sound and vibration, make sure the volume is high enough, etc. (it even says "alarm in 6 hours"!), then I go to bed and wake up at like 12:00 PM. I have a Samsung Flip4, so whenever the alarm goes off for like 15 - 30 minutes without being dismissed it leaves a notification so that I know that I slept through it. It doesn't ever leave this notification. The alarm is just not going off.

"You must have dismissed it in your sleep."

I keep my phone under my pillow. I would have had to sit up, grab my phone, dismiss the alarm, and then put it back exactly where I left it under my pillow. I have no history of doing anything like this. It has to be a problem with the phone.

What is causing this issue? Any help is appreciated!