Skeleton heads

Why does it appear that once a dungeon level is completed, all the rewards/drops seem to dry up?

I have gone through the Tomb of the Cruel Wolf/Wolf 2 over an over and over ( at least 100 times ) and all I have collected is 6 freaking skulls... now it just doesn't drop anything...

How an I supposed to finish the quest if the Skeletons don't lose their heads? :) Hahaha  yes... pun intended!!  :)

And yes, I have gone to other levels too, it is the same... once it is cleared the drops are few, far and inbetween.