Nothing really out of the ordinary, and when assembling a lower, some times you have to clean up the detent channel itself.  Hell, take it one step farther and  often you have to clean up the bottom of the bolt catch spring channel of debris as well (bead blasting media that got trapped when the lower was anodized).

The sector detent is a two step channel, with the lower wider channel taking a 9/64" drill bit by hand to clean that channel up, and the upper step channel takes a 1/8" drill bit by hand to clean it up as well.

As for once the step channels have been cleaned up, if you still have a selector problem with it not being able to turn, then the problem is not the selector or detent, but either the selector detent spring in the grip being too long, or the pistol grip channel for the spring being too short instead.  Hence the spring is coil binding up when you install the grip, and leaves no room to compress more as you move the selector and the detent has to slightly lower down to allow the selector to move into the center groove/ into the other detent position.