The same thing as the OP happened to me today. Literally the SAME problem. The gun has about 1000+ rounds through it. I thoroughly clean it and lube it prior to every outing. This has NEVER happened before. All of a sudden my kid is like "Dad the bolt thing wont go in". Manually using the charging handle gets the bolt back enough to engage the bolt catch. Always on the last round this would happen. Unlike the OP I didn't touch the fcg. Wierd that it started happening out of nowhere.

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U.S.M.C. Veteran