If they fire on the second strike I would tend to think the primers were not fully seated on the first go round. And are being fully seated by the firing pin.
Check your rounds for high primers first.
You recently changed primers and you may need a little more pressure on the downstroke to fully seat them. This is something you can check easily, if none of your rounds exibit high primers maybe it is time for a new spring or you have a dirty firing pin channel.
