Why so few Stevie songs on their albums?

[bwboy]

Especially early on. Christine had 2 hits and Stevie had 1 from their first album, so I've always thought it interesting that Rumours only had 3 songs by Stevie, of which only 2 had her singing solo. Then, with Dreams going to #1 and Gold Dust Woman being such a concert staple, she still had the least number of songs on Tusk. After Bella Donna came out and was such a hit, I can see why the band would only get 3 songs from her, since after that she was focused on her solo career, but I wonder why it wasn't more of a level playing field for Rumours and Tusk?