yesterday my alalrm doesn't work

Today, I also had some problems with the alarm (don't know if it's related to the earlier posts, though).
Please note that the phone was always turned on, I never tried using the alarm, when the phone was turned off.

Some background info:
Bought a used 64GB N9 with PR1.3, did a hard reset, used it for 1-2 months, alarm worked perfectly. IIRC, in the beginning I used the default sound, but afterwards a custom mp3.

Last week, I bought a brand new 16GB N9 with I think PR1.2. When I turned it on for the first time, I updated to PR1.3 and installed some apps (no reset after update). The alarm with custom mp3 worked a few times, but today it sounded up for 1 sec and then it automatically went to snooze. I heard the 1 sec ringing this time, so I woke up and checked what's going on. The device was put on the table facing down (can this be the issue?). I turned it around to face up and after 10 mins (snooze time), the alarm worked correctly.

It would be really bad if I couldn't trust the alarm of the phone.

sorry for the delay
PR 1.3 (40.2012.21-3.232.05.2_PR_232)
.....
Regards,

Olivier