Help needed with my Staff & Cat

Hi All.
I have been having trouble with my staffy Bella and my cat Dave. Bella is 6 years old, we got her at 15 weeks and have never had a problem with her being nasty to anything. We have had cats the whole time we have had her, with kittens coming and going as we used to breed cats. But we have been having trouble with her and the cat Dave(He's actually a 6 month old kitten). She is really nasty to him. We also have his sister, but Bella is fine with her. I dont know what the problem is as Dave has never done anything to her. The are always separated at night(Bella comes upstairs with us and the cats stay downstairs) so I know he can't have done anything to her when we have been in bed.

Can anyone offer any advice??? Thank you.

I would give the cats high places and escape routes is all I can suggest .