I'm in a historic world war game and I'm playing as the Soviet Union. One of my neighbors, Persia, wants to give me his resources and troops. So we've been trying to do that with some success. While usually the trades would go through, occasionally the trade would be refused despite me accepting them. Also, we can't do trades with troops and I don't know why. Is there something I'm doing wrong? Or are there some restrictions on trading that I'm unaware off?