10 reasons your squat might be stuck

• You aren't squatting enough - Whether that be through sets or frequency, the squat LIKES both.  A lot of guys who struggle with the squat don't squat with enough volume or enough frequency.  For guys that really suck at squatting, getting in two squat sessions a week can be a cure for this.  When you do go in for those sessions, keep the intensity (percentage of 1 rep max), in a range where a good deal of volume can be done without destroying yourself.  5-10 sets of 5-8 reps at 60-70% of your EDM is a great place to start.  If you're on the stronger end of things start at 60%.  If you're more of a novice or intermediate lifter, then 70% tends to work.  
• Your quads are weak - For years multi-lifting told guys that quads weren't needed in squatting.  Well, that's true if if you're in a suit in knee wraps and the strength curve has been changed by equipment.  If you're not using those things, then you need quads.  And you need them badly.  To build up your quads your routine should include lots of high bar squats, front squats, hack squats, and leg presses.  Yes, leg presses.  High rep leg pressing and hack squatting are great movements for bringing the quads up to par if they are weak.  If your front squats and hack squats are weak then you're probably quad deficient.  
• You're not stabilizing correctly - If you're wearing a belt, then you need to learn that the belt isn't there to protect your back.  It's there to help create intra abdominal pressure.  And to do this properly you need to learn to push out against the belt.  If the belt is too tight, you won't be able to this as effectively.  If you squat without a belt then you need to learn how to push down into the abdominal region towards the pelvis to create this stabilization.   
• Your cues are wrong - I've read all sorts of cues in regards to coming out of the hole in squats for years and I am often left baffled by them.  "Drive your head back into the bar".  Huh?  "Lead with the chest."  What?  Listen, the HIPS and LEGS are what get you out of the hole in squats.  Your upperbody is there to basically hold the bar tight and statically.  You should be driving with your hips and legs, not thinking about moving your torso.  Mental cues DO matter.  Thinking about what your upperbody is doing during a squat is like thinking about what your calves are doing during a bench press.  So long as you are tight (as described above), then your thought process should be centered about the part of your body that is doing the primary work in the squat.  And that would be your lower body.  
• You're not sitting back efficiently - The first motion in the squat should be at the hips, and then the knees just a split second after.  The problem is, a lot of guys don't know how much sit back they need.  You need to think about the path of the bar in relation to your sit back.  If you sit back too far, then the bar travels very far forward, and moves away from the center of your body.  If that happens then you lose the "power path" the bar should be in.  
• The weight isn't on your heels - This plays in with the previous part.  If the bar is in a good path in relation to your leverages then you should feel the weight on your heels.  That is where you should be driving from as well.  If you find yourself getting the weight on your toes (you've probably felt this a few times) then it means the bar moved too far forward of the power path, and you've lost efficient leverages over the bar.  Make sure your sit back is on point, and that the weight stays in your heels.
• You're trying to be something you're not - At every seminar I do I find people doing high bar squats that are leveraged better for low bar squatting, and vice versa.  Yes, everyone can do both, but you're probably a bit more efficient at one than the other.  Stop trying to force yourself to be something you aren't and figure out which one puts you in a stronger position more naturally.  
• You aren't spreading the load over your lower body effectively - This means you're not bringing in all of the areas of the lower body that need to be involved in the squat.  If you're getting too much butt wink because you're not stabilized through the core, then the glutes get removed from the movement.  If you're not pushing out properly with the knees, then the glute medius and abductors don't get to play with everyone else.  
• You're maxing out too much - This is that dead horse I have to beat so much.  I've seen so many guys over the past year that just try and max out each week on squats rather than actually TRAIN the squat.  Every week I see the same guys doing a max single, and it's either something they have already done, or less than that.  Boost your three, five, and eight rep maxes for a long time.  Quit trying to be a youtube champion and actually go into the gym to TRAIN....not test.
• Your technique sucks - This is basically a simply way to describe all the other aspects related to technique above.  Once you dial all of those things in, and your technique is crisp and efficient then the next step is to do what I just said....TRAIN the squat.