Pull the FCG, and take a look at the postion of the selector detent pin channel in the lower.

If should be dead center bottom of the channel, and not drilled to there, will cause in spec selector to be out of timing when the detent locks it.


Correct dead center bottom of the selector channel for the selector detent pin,



selector detent channel drilled wrong, and lower is out of spec.




On the selector, look at the detent potions in the groove, and should be square clocked to the lever potion.


In some cases, my find a bur in the groove detent edge that may need to be cleaned up, or may have to dull the tip of the pin so it fits the detent channel in the selector better.


As for selector detent, should slide and out of the stepped channel of the receiver, tip of the pin when pushed all the way into the channel should about 1/8 of the way into the selector channel, and on some after market grips, the detent spring channel in them are too deep, and the channel has to be shimmed with a piece of Q tip shaft so the spring is putting the correct amount of tension on the selector to lock into the groove detents correctly.
Note, if the selector spring channel  is shimmed up in an after market grips and binding up in the lower, you used too long of a Q-tip shaft piece to shim up the bottom of the the spring channel in the grip.