I have observed armies with wagon being unsupplied even though they have both river and rail lines linking them to their supply sources. The specific situation are large armies in central Tennessee linked through Nashville and the Tennessee River with plenty of depots along the way.
What I am suspecting is happening is that the three supply iterations can't access supply outside of a 15 region radius of these forces and that whatever this number of friendly regions are they don't produce enough supply to feed these armies.
My question is "Is this logic correct?" or is there another possible cause?
The affect I see is like there is a bottleneck limiting the total amount of General Supply moving to the armies so only some of their wagons can refill but not all. What I have seen on supply distribution doesn't say there are limits to how much supply can be moved but the 3x5 region range would imply it can't be drawn from the whole country just part of it.