First of all, here's to the R9's second strike capability! Some pocket pistols don't have that critical feature.
My next thought would be to ask about the appearance of the primers on the casings that required a second strike. I would think that the "usual suspect" would be some fouling in the firing pin assembly blocking the pin's free movement and full extension through the channel following hammer strike. You might want to try a good cleaning of that area, using a quality solvent and some canned air, followed by a touch of light gun oil. Then dry fire the weapon a few times to clear the channel and see if the problem persists.
Good luck in finding the "gremlin."