I am experiencing a similar issue with all melee attacks with sentinel.
I’ve only played about an hour, so pardon me if this is something that is not a big deal as you progress. I picked the sentinel as my first, and was using the dash attack to initiate fights, but this puts you directly on top of the monster a lot of the time. It seems like if you share the same space as an enemy, all of your melee attacks just miss. If I adjust my character a step or 2 away, the hits will land.
So is it always going to be about re-positioning slightly to do melee attacks or am I missing something?