I am a chiropractor and a strength and conditioning specialist. One of my special interests is athletic development, and I work with athletes in a variety of sports. Your problem is common, and there can be many reasons people plateau. The most common reason I find is that the glutes (most powerful hip extensors) are inhibited by their antagonist, the hip flexors.
A quick check would be to lie on your back with the right side of your body at the edge of a training table (wearing non-restrictive clothing). Drop your right leg off the edge and pull your left knee to your chest. Your knee should remain parallel to the ground and your lower leg perpendicular to the ground. If you knee points up, your hip flexor is tight. If your lower leg is in front of your knee, your quad is too tight. Then move to the opposite side and test the left leg. Confirm these tests by turning face down. Bend your right knee 90 degrees and lift your leg in the air (hip extension). Have someone place one hand in the small of your back and one hand on your hamstring and push down on the hamstring. If you are able to resist, the problem is not in your glutes and you should look at your training. But, if you little or no strength against resistance, your glute is probably inhibited by tight hip flexors.
To remedy this, stretch your hip flexors a lot (up to 10 minutes a day), especially partner assisted stretches (PNF) before a squat workout. Contract your glutes a lot to wake them up. In a matter of weeks your squat will go through the roof if, in fact, your plateau is due to inhibition and not weakness.