Thanks JaPes and Gunslinger for the ideas. I took my lower receiver to my local gun store and they pointed out the issue. When I served in the Marines, I carried the M16 A2 with a fixed stock. My M&P has a collapsible stock. I had to back out the stock to put in the Magpul ASAP. What I didn't realize is that the threads from the stock actually holds the buffer retainer in place (as Gunslinger pointed out). $1.50 later I had a new buffer retainer spring and was able to fix the issue.
Thanks for your helpful information.
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