In my last few games I've had the same issue pop up a few times: I'm allied with a CS, and get the tech that exposes Coal or Aluminum. The CS has some, and may even already have it mined. Even if it is already mined, I do not get access to it. If it isn't mined, and I gift them to mine it, I still do not get it. Then, sometimes much later in the game, they suddenly start giving me the resource. Is there some additional requirement to receive these resources that I don't know about? Does the CS itself need to have hit a certain date before certain resources are sharable by them?