Update to this problem...I had a single convoy running from Vladivostok to Pusan. I deleted this convoy and was able to deploy troops again. Why would having a convoy make it impossible to deploy troops? Convoys lower the supply level of all the attached cities?

<t> This has happened on many of my games. Currently playing as USSR and have captured lots of cities. At some point in the game, it gets to where I can't deploy any units. The deploy button is greyed out. I have many cities and most of them have 0 units on them. So there should be plenty of ...