Does it return with no cable attached? If so it is definitely in the cable. I would guess that it's where you cut the outer. I fyou didn't clean the cut properly, theere's a chance some of the outer is preventing the inner from sliding through.
Or the internals of the lever sticking? - can isolate that option by disconnecting cable from the lever (or loosening it at the lever) and then using a pair of pliers to pull the cable to check the brake?
Maybe cable inner fouling on the inside of a cable stop?
Does it return with no cable attached? If so it is definitely in the cable. I would guess that it's where you cut the outer. I fyou didn't clean the cut properly, theere's a chance some of the outer is preventing the inner from sliding through.