omg i totally agree with u!
IDK why but yeah I guess the unexpected hit of Chuck maybe a reason. At first, Dan and Nate were the lead roles, Chuck was rather a supporting role. But later, C is popular and then he becomes an important role. There are less storylines of N then. Although it can't explain why only Chace gets poor lines while Penn still get plenty of plots.......
Another guess is, Chace wanna do more movies. The less parts he's engaged in GG, the more movies he can take. (U know, tv teen drama isn't a big step for an actor's career.) So, he's gettin' written off. In another word, U can say getting less storylines is his own choice. (sure, he just wants less but doesn't expect less=poor storylines lol)
Some ppl say he can't act, so he's out of the main plot. I don't agree. Well, I admit he isn't amazing as an actor, but hey all GG actors and actresses can't "act" too. It ain't an award-winning show, it just a show that requires good-looking boys and girls to do the acting jobs to attract teens. So I dun think acting is a reason to get him poor storylines.
Hope it helps :)))