VERY rarely are chin-ups weak due to lack of biceps strength, even when the sticking point is that the top/middle.
When that happens I find that in most case it is because the person initiate the chin up with his arms instead of with the back.
The muscle firing hard first, is the muscle that will receive most of the tension/do more of the work. If the biceps fire first to initiate the pull-up they will become the prime mover and the back will, at best, be a supporting muscle. Now, the biceps, even if they are strong, cannot do a lot of weight by themselves. Sure you might temporarily solve the issue by making your biceps stronger, but it doesn't solve the long term technical issue. And since there is a limit toi how strong your biceps will become you will hit a chin-up ceiling very fast.
You need to engage the lats first and do most of the work with them.