Testing for short stroke would be the first logical step. Load a single round and verify the bolt catch is holding the bolt back after the round is fired.
If that checks out okay, I would detail strip and clean the bolt, paying attention to the ejector channel, which can occasionally become obstructed with debris. Check the ejector spring and ejector for unusual wear/damage. Make sure the extractor lip isn't rounded off. Check the extractor spring. Be sure it isn't soft, or an older rifle extractor spring with a blue insert. If in doubt, get a new proper carbine extractor spring (with black insert) such as a
Colt,
BCM or
Wolff.
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