I had a similar experience using my club's ROA. I was using remmy #10 caps which I was advised were perhaps too tight - I wasn't seating them fully home on the nipple so the first hammer strike would complete the seating, the second would then fire it. I've been recommended to use #11 which may avoid this problem and too press each cap fully home with a finger or the end of the capper. Don't have any experience of the caps you were using so don't know if your problems has a similar cause.

Other points that guy's at the club have told me about -if the caps are old/damp they may not fire reliably. Also if there is excess oil around the nipples it can get inside the caps and prevent ignition.

If you find any other problems & solutions, please share!