Not 'feeling' back muscles, Pullups

I love pull ups. I do them almost every other workout. For over 5 years now, I've never been able to do over 13 strict bodyweight pullups in a set. Frankly, I just stopped going for high reps at this point. One thing I do notice these days, that everytime I do pullups or any back exercises I never 'feel' my back. Whether it's rows, levers, deadlifts, I just don't feel that 'pump' on my back muscles. Is there way to know if I'm actually activating my back during these exercises? Wonder if that's the reason I never can go beyond a 13 rep pullup... Edit: I use thumbless grip(tried all chin-up, hammer grip variations). When I perform a max set after a break from workout, I am mostly sore on my arms, shoulders and abs, not much if at all in my back/lats though.