I'm having this happen quite often as well. Miners can't path to ores through fenceposts from the side, and will shut down until you go down and clear the fence.
It might actually be the half block above the post that's the problem, because I've had them get stuck when there's a post in the block right below and to the side. In that instance, it actually mined the plank block that was on the fence post before shutting down.
Really annoying, because the fence post isn't necessarily right next to the miner, and sometimes takes several trips down to find the problem.