Primers Backing Out

[Contenderizer]

I haven't reloaded in many years, so pardon this rather academic question. I have a box of 8X56 Mann/Scho that I purchased from Old Western Scrounger some years ago. After firing a few rounds I noticed that the primers were backing out (I have no means of measuring same, but enough to see, and easily feel when running you finger across the head). If memory serves me correctly, this is an indication of a light load - prehaps a little too light. I have fired other ammo in this gun without this problem, so I assume head space is not the issue. It wouldn't surprise me if OWS has errored on the light side to play it safe ... given the age of these guns. Any other possibilities? Thanks.