My 460 did that at first. The problem was I wasn't seating the primers deeply enough. The first round would simply drive the primer without firing it, and the second impact would never fail to fire them. The fact that all the misfired rounds fired the second time is proof that you've got the same problem. Seat the your primers as firmly as you safely can. If you have a handheld priming tool, you'll feel when the primers seat properly.
"We are only as strong as we are united, as weak as we are divided." - J.K. Rowling