If your alarm doesn't seem to work, the first thing you should check is that it was enabled for the day when it didn't work. As you can select on which day it should wake you up, it is possible that you have accidentally disabled it for a day when you needed it.
If everything is good on the setup side, it probably is a battery issue. While there may be enough battery for the watch to work properly as far as giving time goes, the fact that it's a connected device means it requires a bit more energy that a regular watch. When your battery no longer has enough power, the absence of alarm is usually the first sign. Go see a watchmaker so that he can replace the battery.