I have 3 cats, when brought home my 13 week old Golden Retriever it was intresting how she interacted with each cat. My calico (who is 4 yrs old) would just stand in front of Mandy (puppy) and look at her so Mandy would just look back. Sammy (5 yr old Maine Coon) would always run from Mandy so Mandy used to chase Sammy. Buttons ( 4 yr old tortie) hiss and growls at Mandy so Mandy steers clear of her.

You can keep working the "leave it" command and eventually it should work, but keep in mind the cat is a fun moving thing to chase so it will be hard to convince the puppy to stop chasing the kitty. My suggestion would be keep practicing the "leave it" command with other things practice so much that "leave it" is a reaction and he almost automatically leaves it before even thinking about it. In the meantime, try not to make a huge deal about when he chases the kitty ( so it doesn't become a form of getting attention). Does your cat have claws? If so the cat will let the dog know if he is going to far in the playing. Hope this helps, good luck.