Help! Safety Selector Problem

[montana]

They were the only detent pin and spring in the bag with the selector, so I would assume so

I would remove the pistol grip part way to relieve the selector spring pressure and see if this helps. If this helps I would check to see if your selector detent tip looks OK and the spring is not bent. The selector detent is different than the take down pin detents which you should be able to Google an AR-15 schematic to tell the difference. Make sure your selector is installed all the way so the selector detent fits in the grooves correctly. If this doesn't help well try some other ideas.