I noticed this yesterday which seemed odd at the time. I was rushing an ideology and managed to get industrialisation first. I quickly looked around for any CSs with coal bought one but quickly realised that the coal they had wasn't within their borders. So I bought Belgrade (see attatched) and still nothing. Many turns when by and I couldn't figure out why I didn't get the coal - I couldn't pay to upgrade the resource because it was already mined. Ultimately I was beaten to the prize of early adoption by then I did start getting the coal. I can only guess that the CS didn't have the tech and couldn't share with me (even though it was mined?). In another game I've been in in a similar situation where I had the option to pay to upgrade the resource and, voila, I got the resource. So do you have to wait for a CS to catch up in tech before you can share their resources? Or is there a discrepancy between if you pay to upgrade a tile and this case (assuming you're the tech front runner). I raise this point because this hasn't happened to me before. Each time I've needed a resource from a CS and payed to improve a tile I've had it immediately.