I'll echo what Big Swede said and suggest GMs in addition to rack pulls as good assistance exercises in general. (You guys were referring to rack pulls when you said "partial deadlifts," right?) Rack pulls will allow you to pull more weight than your standard and work on your lockout. I'm also partial to deficit deadlifts as an assistance exercise.
But, what I really want to says is that if you get the weight above the knees, it seems to me that the problem is upper back strength. You posted that you do rows as well. Are those DB or BB? I think working extra hard on rows can help bring the upper back up to par and help you lock out that weight.
I missed a deadlift at the same point in the movement you did and my solution is to make sure I continue follow my squats with GMs, my deads with deficit deadlifts and deadlift holds, but to really KILL my rows, either BB or DB, when I do them. I even got a pair of straps in order to really hit my back as much as possible on BB rows.