Pull up help

Hello! Recently started working out and I've been consistent for 3 months now, last month I've unlocked pull ups and I wonder if it's normal to somewhat not feel it in your back?

It's not that I'm using my arms more or my form isn't right, I've already recorded myself alot doing pull ups and my form is good.

But my reps are decreasing, for example my first set is 7 not assisted and my last set is 5 assisted bands.

Idk just curiosity, I might be doing something wrong but idk