i went down in weight to make sure im not hitcing with my shoulder. look around, you will see guys doing dumbell/barbell rows with weight they cant handle so they incorporate this jerking movement to get the weight moving. the problem hers is, they lose the resistance on the lat. when they do this, the only time emphesis is on the lat is when it is stretching at it hangs. lol. try decreasing weight and holding for one or two seconds at the top of the movement/as you squeeze your elbows towards each other. you will hv to decrease weight at first, but your lats dont know how much is on the bar. keep the emphisis on constant resistance on the target nuscke group, and the numbers will come.