I suspect that your frame is "out of square", that is, you don't have right angles in all 4 corners of the frame. That distorts the rest of the frame. What you describe is very similar to what happened to me after I moved my frame from one side of the room to the other.
It took me a while to figure it out. After exhausting all other possibilities, I finally looked at the frame. In the center front of the frame there was a bulge. None of the corners were at a 90 degree angle. It took some doing, but with the help of my DH, the frame was straightened and my quilting improved.
By the way, my machine is computerized. The little robot that moves the carriage around travels on the tracks on the frame, therefore, it followed the frame/track curve while quilting.