Some lifters have trouble hitting their backs, even with back exercises. Often, the biceps take over. The unilateral lat pulldown allows you to really focus on the back one side at a time.
Do not fully extend the arm at the top of the movement.
We all go through periods when pumps elude us. Frankly, these periods really suck. And worse, they seem beyond explanation. It's a bloody shame, considering how motivating enlarged, vascular muscles can be.