Combat Trim has nothing to do with your flaps. What controls the flaps are the keys q and w. What controls whether or not you may drop your flaps is the speed of your aircraft.
The P51 for example will allow you to use flaps utpo about 300mph, these are called combat flaps. Planes like the 190, 109, La5, La7, Yak have flaps, but cannot be dropped until about 150-180.
SO you arent doing anything wrong, but I suspect you are just going to fast to get the flaps down.
Nim