Snowman,
I've never had that problem, but here's what Scott Duff has to say on the subject:
first, I assume you've tried several clips with the same result. If not, get some new clips,
The second most common cause is a broken clip ejector. Its part of the trigger group.
The third cause could be problems with the bullet guide and follower arm, which are part of the receiver group.
Do your self a favor anf get Duff's "M1Garand: Owners Guide". I'm sure he has a web page. Good luck.
Jim