The barrel needs to be re-indexed to the receiver.  When the barrel is installed, it's easy to allow the barrel to turn with the barrel nut as you're torquing it down, even though there's an indexing pin that's supposed to prevent this.  The pin can get bent easily.  Anyway, any AR 'smith can correct this in a few minutes.  Kurt's Kustom can take care of this, and get you within one or two clicks of center.

-Troy

Retorquing will probably fix this unless the barrel is bent. . . . .

Straightness gages are rare outside the military, but observing the light reflections down the bore from several angles can offer some clues.  Should be an even reflection.