A common mistake i've noticed, is that some people don't take into consideration the infrastructure. The difference of supplies that can pass through a 10 - 20% Infrastructure province to an 90-100% province is stagering.
I agree with Vhorthex and others who argue that aspects of the supply system are being overlooked by players. Huge swings in supply demand may be explained by units trying to replenish their 30 day supply-quickly.
Blitzkrieg advances through "garbage" territory is going to create supply problems. Remember that when a province is occupied, some damage to infrastructure occurs, more if the the province was defended.
I wonder if some folks are using a civ4 style "stack of doom" to conquer territory, and then are surprised when supply problems appear.
I suspect that poor attachment and HQ management may also be responsible. Even rear echelon forces will require structure, and their HQs must be within radio range.
Considering the ease HQs may be created, perhaps garrisons in home and occupied territories require a full command structure. A good position for old guard tools.