I used to have trouble sewing 1/4 inch, and I finally figured out it was the 1/4 inch foot I was using. I was using the foot with the little piece of metal hanging down. In hindsight, I think I was jamming the fabric up agains that too much, causing a slightly larger seam. When I changed to a 1/4 inch foot without out that metal piece hanging down....a clear foot so I could see through it, I found out that worked much better for me.
Maybe you are doing a similar thing? Or maybe you just need more practice. (I needed more practice also. I was pretty impatient with myself, expecting to conquer the 1/4 inch seam quickly because I had been a seamstress all my life, but I had never even thought of a seam smaller than 5/8 inch. I was just very impatient with myself.)
Dina