The two main issues I see are as follows: 1) You aren't leaning over enough and letting your knees go forward enough in the beginning of the reps (see side view video); you are too vertical with knees not bending enough in the beginning. Think "lean over, butt back" in the beginning to get the bar over mid-foot sooner rather than later. 2) You are too deep on most of the reps which is contributing to your knees punching forward/sliding in the bottom. Aim for hip crease just below parallel. The combination of these two errors might be making your knees feel not so great at heavier weights.