I have heard it both ways. I've heard that Joss felt SMG wasn't grateful enough to him, because Buffy is what made her a star. And then I've heard that SMG felt Joss wasn't grateful enough to her, because it was her acting that made his show popular. Could be a little bit of both, I guess, although SMG doesn't seem to have a diva bone in her body so it's hard to see her making a stink about not getting enough gratitude from Joss.

Either way, it definitely seems like they didn't get along. If nothing else, I find it telling that Eliza got a career intervention from Joss because it hurt him to see her in crappy horror movies when she's "so much better than that," whereas SMG, who has far more talent, didn't get a similar intervention when she did a string of equally bad horror movies.