Here's what I did to reestablish how to get a 1/4" seam when I started quilting again after a many year break. I cut 4 pieces 1.5" by 4", then I sewed them together, pressed them and measured the full width of the resulting piece, which should be 4.5" exactly. Then I cut 4 more and did it again, adjusting for whether I was under or over. I did this several times until I knew exactly where the right spot was on my machine and foot.
It can be hard to tell if you are slightly off with only one seam, but when you multiply that you multiply any problems and it's easier to tell if you are off a bit.