Second what Peter has written. As an old school powerlifer, I would recommend you "sit back" more. As Peter mentioned, you're knees and shins are going forward. I, personally, recommend that people not only get their shins as vertical as possible that they overexaggerate the weight distribution onto their heels. I've even gone so far as to recommend to lift the toes during the descent to force the body to sit back with the shins near vertical.
Hope that helps?
All the best,