Did you introduce them the right way? You know, have one cat play in one room than let the other cat come in and get the smell with the first cat not around. Then do it a lot and get the smell around the house so the cats can get use to the smell of the other.

I did that with my two. Then I had my cat look at the new cat in it's carrier. Then slowly let the new cat out so they could finally touch noses. It does work. Patience is all that is needed and a no nonsense attitude that they WILL get along.