one thing that helped me was imagining ure hands as hooks
if ure using straps grip it so ure hands are like hooks and lift with the elbows.
when doing barbell rows, lower the barbell so ure shoulders seperate and when u lift imagine lifting just with ure elbows, the second ure biceps start to contract ure not hitting ure lats efficiently.
as mentioned lower the weight and "feel" the rep... the mind-muscle connection for back is hard to grasp its not something u can see flexing in the mirror. if ure working out with someone tell them to put pressure on the lat while ure doing a rep u can feel the contraction better when u do that.
when doing pull ups i found pushing the chest out and keeping ure hips under ure body help me hit lats more.
the hardest parts on my body to grow are lats and traps, one thing thats help it is rack pulls they definitely my upper back more and there's a few articles on here that say rack pulls help stimulate the upper back when done correctly.
also some people are real big on static holds in the pull up position and negatives, ure gonna have to experiment to see what works for you and what doesn't...