Supply

[Uboat47]

Can someone explain supply to me. I don't seem to understand what makes a unit out of supply or not. I have had large swaths of units surrounded not every hex was covered, but they were still surrounded on all sides by my units. I wait a turn and they're still not out of supply. I'm unsure why, and its bugging me to the point that I might not want to continue playing. I have looked through the manual a couple of times and there wasn't really any sections that related to this question. PS the game I was playing was Smolensk 41.
Regards Scapa