If the gun does not misfire with factory ammo I would take a close look at my reloads.
Maybe the primers on some of the reloads were not seated all the way. If this is the case the first strike will seat them and the second strike will fire them.
What primers are you using? You might want to try some with softer cups.
Federal primers are known for soft cups.
Regards
Bob Hunter
www.huntercustoms.com