Supply units can only move on roads, tracks and railways so many areas of Angola are quite simply impossible to attack. Next turn you can move the supply unit, usually by an airlift. Then it can take 1-2 turns to get the supply unit to the front. The FNLA is very fortunate in the beginning because both of their airfields are very close to where the initial action is going to take place, so they get into the fray quickly. Combat units move before supply units move in the Turn Sequence.
Combat takes place after the supply movement.
So planning is necessary here. You can of course save up supply for some major offensives but as it is useless in defence, any big pile of supply units is bound to attract the attention of you enemy. The availability of supply is also important. It has already been predetermined and is listed on the Reinforcement Chart. This makes the FNLA very weak during the last half of the game, regardless on how well they did in the first half.