If you haven't already done so, I would take the bolt out and verify that there is a firing pin present. If so take the bolt apart and clean it. While it's apart check the firing pin to see if the tip is broken.

If you don't know how to disassemble the bolt look in your owner's manual or on youtube. The first time is a bit intimidating, but after that it is pretty simple.

Find a picture of an AR15 firing pin online and compare yours to it. Of course there could be other issues, but this is a good place to start.

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Or something like that . . .