Just as PraiseLuka said, it is rare but it happens. I know some occasions I would miss SS because I did not have the time to wait for the refreshes, but there was one occasion where I waited and did not get an SS (once in the entire time I have played Naruto Online).

Hopefully, we don't encounter too many of such bad luck. I will also forward your feedback about SS missions.

~Jib

Update: I have just asked the team and there was no change with the rate of SS missions being available. I will continue to update you if I learn more =]