When you say you've been stuck at 90kg...
What exactly do you mean?
Have your reps been going up? Or have you literally been stuck at 90kg, hitting the same reps each workout, for the past month?
How many times have you squatted and not progressed? My workouts fluctuate all the time.
If your reps are going up, your making progress. If you've been stuck at the same reps, at 90kg, for the past month. You're obviously doing something wrong.
It could be bad recovery? As in not eating enough/sleeping enough/too much stress. etc. Although if your other lifts are going up, I doubt that is the main cause... Although it may be a factor?
It could be as simple as not pushing yourself hard enough when you squat? Are you going balls to the wall and training close to, if not to actual failure each set?
Could be really weak core strength? Or other weaknesses? You're only as strong as your weakest link.
Like I say though, I'm **** at squatting.