I've had this problem before with a previous project but never got the bottom of it and ended up just giving up. I've started making a character made from various pieces of geometry which I've merged together using the vertices. Now it's basic fleshed out form and all joined together I wanted to convert it from a polygon to a SubDiv to add more detail. The problem is it won't convert. I've tried messing around with the maximum base mesh faces, maximum base edges per vertex options but it doesn't seem to make any difference. I'm guessing this problem could be caused by some dodgy geometry when I was joining it togehter but it looks fine.
Any ideas of what the problem could be or advise on how to go about fixing this problem.

Cheers