You're not really supposed to keep failing at the same weights forever on StrongLifts. If you fail to get your reps three workouts in a row, you're supposed to do a deload (cut the weight 10% and work back up). Fail three workouts in a row again, do another deload, and now you switch from 5x5 to 3x5. After the next two deloads, you switch to 1x5.
I did all this, except I was hesitant to switch to 1x5. You're supposed to be squatting around 300 when that happens, and also I just couldn't bring myself to do only one set. After I tried 1x5 for a couple weeks, I went back to 3x5, and just said screw the deloads.