Pull the grip off, carefully pull the spring and retainer (detent) out of the hole. Look at the tip of the detent. It should be a rounded off cone shaped and no burrs on it.
Remove the ambi safety lever from the lower and look at the retention groove. It should be smooth end to end except for the drill spots on each end that the tip of the detent goes into. There may be a ledge or burr right before the drill spot where it was pushed up when they drilled it (If there is, remove it).
Also check the back side of the ambi lever or burrs and rough spots.
If these all look good and the sharpie shows no interference, you can try removing 1 or 2 coils from the detent spring to reduce tension and let the safety move easier. Do not reduce the tension too much or the safety will not stay in either the on or off position.
Joat