Last night I practiced heavy squats for lower reps for the first time in a long time. It’s taken me a really long time to get my form to an acceptable level. I have very bad dorsiflexsion (ankle mobility). I have to squat with lifting shoes on as they have a small heel which helps bring my knees forward when I squat. If you are having problems with your form then check out Squats University. It’s really hard sometimes to know what exactly is causing you issues but at squats university they break it down really well.