Resources will only pop from mines that are being worked. That is, hills outside of city fat crosses will not pop resources, and mined hills inside city fat cross will also not pop resources if you don't put citizens on the mine.
Generally, when I played Settlers games, I had popped about two to four resources per game. On Noble, it drops to one or two.
Some people mistake hidden resources for popping. These "pop" when you discover the corresponding technologies. Because the messages are the same, it's easy to think you popped it out of chance.