Handloads are often the culprit of sticking cases. If they are not handloads and the cleaned chamber (with no oil as others stated) does not remedy the problem, I would then check the primary extraction. Which is often the cause of stuck cases.

Do you know the complete history of the rifle? Are the problems recent? Has the bolt handle been changed? Is the BAS tight? (Little things like the bolt assembly screw being a little loose has caused this for me).

Do a search on here for PE. There are a couple of easy tests you can make and a few suggested cures.

Good luck,
Randy