i went out hunting this morning with my ar, and when i tried to clear the chamber of my gun, i could not. i briefly reefed on the charging handle with no luck. and then i gave up at the time, thinking it was to cold outside. later when i got home, i pulled the upper off and tried some more to pull the ch/b/bc out, thinking the warmer temp might do it. no luck, i finally ended up tapping on the bolt, to get it loose. any ideas as why this happened?