This turns up fairly often on the M1 rifle. These rifles are aging and usually well used and things need replacement.
A fast test to determine the cause is to stretch the clip latch spring and test fire. If this corrects the problem, immediately replace the spring, since stretching it ruins the spring and it quickly fails.
Buy a new clip latch spring from Fulton Armory or Orion 7.
If this fails to correct the problem, replace the clip latch itself and possibly the bullet guide.
Most common cause: Weak clip latch spring.