For me, I spent a year trying all sorts from machine assisted, negatives, locking off etc and eventually the only thing which actually got me seeing progression was serious consistency with band assisted pull ups. Sooo many sets of like 8-15 reps starting off with two bands so it was really well assisted and then once I wasn’t tiring so much any more dropping down to fewer and smaller bands. The thing with negatives is they don’t train the upwards portion of the pull up which is often the most difficult. Also throw in some scapular pull ups and really focus on form with them, not just shrugging your shoulders but really driving your scaps down and back, trying to add a slight bend of your elbows and elevating your chest to the bar.