When you say it flys badly, what do you mean? What areas of stability are poor? How does it respond to control input? All other things equal, as you change an airfoil from a flat bottom to a more symetrical shape, the AOA needs to be increased to generate the same amount of lift at a given speed. How you added the shape to the bottom of the foil also makes a difference in the characteristics. You might have moved the max. thickness to a different spot along the chord? This would affect CG and lift.

If you have any pictures that might help us... Especially a profile of the foil and incidences of the wing and tail.

Also what is the weight, and wing area?