The Germans did develop a drop tank for the Me 109 prior to the invasion of France. There were a couple of problems with its use:

1. The tank was made from 'non-strategic' materials...ie wood primarily, and had serious problems with leakage. They also tended to deteriorate quickly when left out in the weather.

2. Most Me 109 in the inventory were not equipped to take the tank having neither the mounting point nor the associated piping etc., to use it.

3. The whole program had low priority so no one was really taking a proactive role in making sure these would be available in the near future.