Sometimes horror sequels are as good as or better than the first. Boogyman 2, Friday the 13th part 2, and Creepshow 2 are examples of horror sequels that work. It is a matter of opinion, of course, but Friday the 13th, for example, spawned several following films that were based off the second one.
I liked Insidious 2, but I'm not sure it was better than the first one. The first one started off great, but the second half did not do the first half justice. But to answer the question, yes some horror movie sequels work. I could come up with plenty of sequels that didn't work (Ring 2, The Exorcist sequels, etc), but one sequel that did work was Dawn of the Dead. I love Night of the Living Dead and consider it one of the best horror movies ever, but Dawn was just a little bit better. It even made a great remake.