I suggest you take the gun to a gunsmith and have the headspace checked. If that is OK, then check the chamber for rust, dirt, etc. You might be able to polish the chamber using something like 600-1000 grit emery paper on a wood dowel, but that would only be after you find the problem.
Case expansion is pretty common and you often won't be able to re-insert the empty case into the chamber until it is resized, a normal part of reloading.
Jim
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