What load are you shooting? It could be that it is not cycling the gun fully (look at my thread on model 1 sales problem). There are several fixes for this.
Also, it is possible that your load is too hot, and the extrator is ripping through the rim. You could tell this, very easily by looking at a fired case.
You could also not have enough extractor tension. This can be improved by replacing the extractor spring or adding a D-Fender ring under the extractor. I have a D-Fender, but have not put it in either of my AR's, as they don't really need it.
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