Something could be wrong with that 7th Arduino board.
If 6 work all the time, and 7 almost never, that is weird. Maybe a bad wiring or a voltage regulator at its limit.
Do you have a multimeter ? a scope ? logic analyzer ?
Did you know that you can fix a bad link in your post ?
Sometimes I edit my reply and someone is already answering my reply that I'm changing. Try to avoid that 