Use Federal primers. My first Contender was a 7-30 Waters. It had the same problem with my first re-loads. I had obtained that brass as once fired. I messed around and messed around trying to get them to shoot. Then I bought a box of Fed factory ammo just for the brass. Not one misfire. That put my head scratching on what was the problem. It then hit me that Federal had the softest cup of the primer manufacturers. Federal primers and a few thousand rounds later and still no misfires.
My 35 Rem contender has never misfired and it has been fed only Federal primers. I think the "head space problem" in the Contender is way over rated. It gets blamed for a lot of things it has nothing to do with.