question on Supply trks/depots

[mac5732]

I'm playing the Japs 1931 against chinese when i picked a supply trk it came in with nothing loaded on it. when I put out of ammo units with it, none ever recovered any ammo, so i tried ammo dump, same thing, came in without any ammo and no units would replace their ammo. Is there something i'm missing in order to get them to have supply when they come on the board in order to resupply my units ammo, tks in advance ??????