The first two seasons of Kirstie Alley on Cheers had Sam and Rebecca as the main focus on the series. The minute Sam stopped chasing after Rebecca however, it's as if her character became inconsequential. Basically, she went from a co-lead like Diane was into a supporting character. Even Carla had more screentime in those last years.

I did notice that too. When Carla's running gag "Seriously, what does she do here?" started to come up, that's when I think we see less of her as if she was useless in the bar.