3 was ok but not as special as the first 2

The reason they fall off is because the true meaning of why Pinhead (and the cenobites) are even there are drastically changed. They're not originally villains. "You called, we came" as they only arrive when the box is opened. "Angles to some, Demons to others" since Frank opened the box originally to find some sexual pleasure and ended up getting the type of pleasure he didn't sign up for because he didn't understand what the box held. They were just there to answer his request even though that wasn't what he intended. The book is awesome at showing this also.

Part 3 was just Pinhead trying to take over the world for some odd reason and the other sequels were just a cheap cash in on a cool looking character.

If you're looking to get into more Clive Barker check out his Books of Blood series. Great reads.