supposed to happen.
Its caused by the floating firing pin in the AR design. Long story short, the forward momentum of the bolt closing carries the firing pin forward until it meets something solid, like a primer. It is rarely enough force to fire the round and no fix is nessesary as this is how the rifle is supposed to work.
However, be warned, if you reload and your primers are not properly seated, as in protruding beyond the rear of the case, it can cause premature primer detonation, resulting in all manner of problems.
Still not much of a concern though, just reload properly and you will be fine.