They're probably flying in some fuel, but there is a limit to how much they can do: a Wellington is a high-tech heavy bomber of the period, and a Harrow is about the best transport available. Throw in the fact that it's the middle of winter with limited daylight and the weather is probably also putting a severe crimp on operations, and there is a limit to how much you can do. Remember we're on the cusp of a transition from rail and horse-drawn logistics to motorised ones, and that means the RASC and their French equivalent are simply not up to the task they're being asked to carry out. If the Germans were in any state to fight, this would be a disaster - the military professionals know this, but also know that the Germans are in real trouble and that there is an overwhelming political drive to relieve the Poles.