Each monster has a number of properties which can default to either set or cleared. Dropping treasure is the default for most monsters, as is the sinking through the floor, but those values can be changed by the designers.So if a monster of a type that usualy drops loot doesn't produce any then it had to be set that way at some point. This might mean it was done intentionally, but it also could occur if the level was populated with an early version of the monster which didn't have the flag for dropping loot. I fixed a few monsters with similar problems but none for not dropping treasure. If you give me specific locations I can look into it.
