Think about this, during rows and lat pulldowns one can always control the resistance. Not so during pullups. You are a slave to body weight static resistance. Not even talking about adding additional weight from other sources.
It's no wonder other smaller muscles groups get tired before the lats get enough work to grow.
This is one reason, I suggest variation on pullups to shift emphasis away from those other smaller muscles and maybe find the best variation that individually target the belly of the lats best.
A time consuming process indeed, but worthwhile imo.
Thread: Lagging Lats