what i have found with one of the ladies i quilt with..
she started out the seam,tight against the 1/4" foot bar..by the time she got to the end of the seam, it was wide..like she relaxed and just let it go.
i don't watch the needle, i watch where the seam is or the right edge of what i am sewing..
what kind of machine do you have? and are you using the 1/4" foot? are you able to move your needle to the left or right to find the 1/4"?