I agree with checking your scant seam allowance as PaperPrincess suggests. Scant really does mean scant in some patterns. I use moleskin or another physical guide to ensure my seams are exactly the size I want, as I was never able to achieve consistent "scant" by using a 1/4" foot. Also, the thread you use can make a difference. It has been much easier for me to achieve consistent "scant" 1/4" seam allowances after switching to Aurifil 50wt 2-ply thread, which is a finer thread than the standard 3-ply thread I had been using.