Ezra fails to attack with "watch & learn" despite no debuffs? Why?

I am using him constantly and i've never saw him just staying there doing nothing. The only scenario that comes into my mind is if your target is getting killed by the character who assists, therefore there is nothing for Ezra to attack. Otherwise, debuffs like daze prevents assisting, but you said you have noticed no debuffs.

I suggest you to test this a few more times, and if you see problem is persisting, go record it, then post it on Bug Report sub-forum. You may get a symbolic reward as well if you're right.