Alarm clock not ringing sometimes

Over the last years I think I experienced two cases when the alarm clock was not ringing.

One time it did not ring at all and today I noticed that after snoozing once the next time only the light ring was flashing. When snoozing again, I got the audible confirmation and from then it rang again properly.

The first occurrence happened on an Echo Dot connected to and external speaker via AUX (I am rather sure the external speaker was not the issue there, but maybe the output channel is related).

The occurrence today was on another Echo Dot also connected to external speakers via AUX. This time I am sure that the speakers weren't the issue since I got an audible response when snoozing.