Wow, how did you find this 9 year old thread?
Anyway, what ammunition- specifically?
Is this a rifle that you or someone else built? Is it a factory built gun?
Is this gun new, or is this something that just started?
Is the BCG lubricated? Doesn't need to be sloppy wet, but should at least glisten with oil.
DPMS chambers have been said to be somewhat rough, and benefit from a mild polishing. I put some rubbing compound on a chamber mop, chucked it into a cordless drill, and went to town on mine. Other than that, it sounds like it is short stroking- the bolt moved enough to open the dust cover, but not far enough to extract or load.
Likely culprits are insufficient lubrication, heavy buffer, heavy spring, insufficient gas from the gas block, and underpowered ammunition.