Another thing to watch is how your bench is constructed, and if it is flexing too much for a good, positive seat of the primer.
When I began using my 550 I had the exact problem you are describing. After going through all of the "checks" to make sure it
was put together correctly and getting the same results, I took a look at my bench, and found it less than stellar, as it had only
been used for a single stage previously.
I solved the problem temporarily by holding onto the press in the rear and pulling forward slightly, as I pushed forward on the handle
during the priming operation. Solved the problem for me.
Good Luck