He asked if you got first hit because then you would have gotten a chest for sure (red mob instead of purple mob),
My fc had the same Problem at the first time running Diadem but it seems to have gotten better since them we are not sure what exactly the criterias for a chest drop are. I saume it is a mix out of dmg done and rng, as sometimes we get a chest after doing almost nothing and sometimes we are there for the entire fight and get nothing.