Two questions
1.) Did you save the file after you made the changes
2.) Did you move the the gate or the circle with the plane in it, ???
 In order to move a plane further from  the gate move the circle with the plane in it, not the gate itself and rember to save before you quit AFCAD 8)
You may have to move it a good bit in order to get it where you want it. I have yet to figure out exactly how the circle corresponds to the actual postion of the plane. I know that the dot represents which way it faces, but am not sure if that is the nose or the nose wheel of the plane. 8)