Spot 5, 2 notes and the alarm stops playing

[LeKeiser]

Hello everyone,
I have a weird problem. I have a Spot 5 in my bedroom with a few alarms configured.
I don't know why, but sometimes, a lot of times actually, the alarm starts and stops after just a couple of notes. "Dim Dam Du..." and everything is quiet.
I have reconfigured the alarms. I have changed the the time, the notes. I have deleted the alarms. I have created new alarms. I have factory reseted the Spot. But still, sometimes, most of the times, the alarm kicks in and then stop after 3 or 4 notes.
I don't know what to do to fix that annoying problem...
If anyone has an idea

Thanx for your help