After I hit ~150 mining, I've noticed that veins can only give 1 skillup max now. For instance, ever since I've been able to pick at Iron, the first pick I do always results in a skillup, but the next 2-4 picks only give me the ores/stones/gems.
So, is this a game mechanic (only one skillup per vein past level X mining), or is this just really, really, really bad luck?