She's kind of a loser

Her acting choices have been, to say the least, pitiful.

She lands the lead on Boston Public only to leave the show for........Nip/Tuck.
And on that show her character pops in and out and has a hostile and irritating dispostion (with her saying *a--hole* all the time as if she sounds cool saying it). So she walked away from a lead role to take a smaller role in addition to nothing roles on Heroes, Friday Night Lights and Prison Break. None of these roles have been worth squat.

Finally, her character on Glee is clearly the most unlikeable, the most dead weight of anyone on the show. She doesn't sing, she doesn't really advance the main action of the plot and her only real purpose is to drag Will down.

This string of bad luck must be karma for orginally leaving BP. Serves her right, not all actresses can be in A-material and this is what she gets when she's picky.