The selector detent pin should install and drop out free from the receiver channel step channel.

If such is not the case, then may need to take  drill bits to clean up the two step channels, so the selector pin can free float in the stepped channel.  For top of step channel, use a 1/8" drill bit by hand, and for the lower channel, use a 9/64" drill bit by hand to clean them up, so the selector detent pin is not binding in the step channel/will free drop out.

Next is the pistol grip in play, with some of them having the selector spring channel too deep in them, and the spring not applying the correct pressure to the detent pin in the first place.  On such pistol grips, easy enough to break off a piece of Q tip shaft, to shim the bottom of grip spring channel up, so spring is applying the correct amount of pressure selector detent in the first place.

Don't have the depth spec for the channel on a usgi grip off hand, but should be easy to check from a working good grip, to know how much the aftermarket grip spring channel needs to be shimmed up. Also, double check the alignment of the grip channel to receiver channel, so may be off enough to cause the spring to bind between the two, or grip not seated correctly, to cause the spring to bind between the two instead.


https://www.ar15.com/guides/files/tutorials/parts/lower.html

As for selector, check the groove channel in it for the pin, as well as the tip of selector detent pin as well.  On the selector groove, make sure that groove between the detents is not rough, and if detent pin tip is razor sharp, then run it down some 400 sandpaper to slightly dull it. as for selector and lower receiver channel (and the rest of the FCG parts),  should be lubed with CLP before they are installed, and after a few hundred rounds, pull them to re-clean and lube them before reinstalling, to removed the powderized parkerizing and anodizing that has worn of from use.

Yes and no.  
Yes if going to pull the FCG parts after a few hundred rounds of use, to remove the powderized anodizing and parkerizing that the white grease is going to collect.

No if using CLP to lube the parts, since cleaning the lower with CLP , and the added CLP during additional cleanings will allow the powderized anodizing and parkerizing  to be flushed away if not pulling the parts after break in isntead.

Bottom line, the FCP should be lubed before installed, including pins and channel, and problems happen when the parts are installed dry instead.