Your back angle during the descent is more vertical than during the ascent. It changes just after you start to ascend and you're likely losing some force at the bottom. Try to get more horizontal as you descend so that you keep that angle and stay TIGHT at the bottom.
Also, notice your left knee as you ascend. It is slightly moving inward. Try forcing your knees out harder to keep it over mid-foot.
Another thing that I noticed is that you appear to have some lateral movement (assuming the rack is vertical). The bar/plates seem to shift to your right on the way up. I'm not sure what's causing this, but if there is lateral movement it needs to be addressed.