I have the same problem.
I think the transports fly, because there is more fatigue each turn, but they do not deliver any supplies to the units if there is enemy in the same hex.
Also - the amount they paradrop is meagre... I don't know how to calculate that, but about 80 Tabbies can transport this way about 100 supplies (or so) each turn. Can anybody tell how to calculate the exact number?
If enemy in the hex, exact number is ZERO.
If you have no enemy in the hex, take load capacity of the plane in pounds, divide by 2000, multiply by number of sorties/planes that fly (which is generally unknown, but could be up to the entire number of planes in the unit.) This gives you the number of supply delivered.
So, if you have some bombers with capacity of 2000 pounds, and there are 16 planes in the unit, you could deliver up to 16 supply per turn... of course, if only 1 or 2 planes of the unit fly, you would get only 1 or 2 supply/turn moved.
i don't recall what the load capacity of a Tabby is off hand (and i would guess it depended on the mod, etc.)