Old trick time. All wheels will develope a flat spot if the plane is sitting on them for any length of time. The foam tires just do it faster. If you stick them in the freezer it sometimes removes the flat if it isn't too big. Some people soak them in water first? If you have some old foam tires around you should give it a try. When I had planes sitting on the floor in my old shop I made up some wood blocks that looked like wheel chocks that fit under the axles and kept the tires off the ground. When I went to air shows I used these chocks to keep my planes from moving around in the wind too.