New or reloaded shells? If new, then it could be a firing pin that's too short or something (like grease) is keeping it from coming out all the way. Has it ever been cleaned? Reloads, check to see if the primer is set too deep.
Do you have a nice deep indent in the primer on all shells that fire and the ones that didn't?
John 3:16 & 17
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. For God sent not his Son into the world to condemn the world; but that the world through him might be saved.