burgingham wrote:All caves in an area are on the same level, so it might be that the mine is actually in a cave nearby and not in the one you are prospecting.
At a 24% reading, this is almost guarenteed.
Try trace the direction and keep searching until you 'run out of cave' to find the genral direction in which the reading increases. Then go look for another cave entrance relatively nearby and keep prospecting. Keep a block of wood in your inventory, since when you get very close to the mine hole (reading >97%) your toon will walk towards the mine hole and smack a construction sign into the ground at the mine hole position. A mine hole takes 100 blocks of wood and that's what the 1 block of wood is for, so you can mark the position via the construction sign, and then go get heaps of wood to complete building.
Finding a mine in a cave is almost always a bitch, so best of luck.