I just wrote this yesterday to someone else who was having a similar problem.
http://www.quiltingboard.com/t-112724-1.htm#2925138
You might very well be sewing a perfect 1/4" seam. But it you're still coming up too short (or too long/wide), then your problem is either in the cutting or in the pressing.
And a 1/16th off in the cutting, not pressing the seam properly here and there.... then you sew a couple of subunits together and your block is short 1/4". Not a big surprise.
Those other steps are just as important as the sewing. And that's not quilt police propaganda. ;-)
As I did in the thread link, if you can get your hands on "The Art of Machine Piecing" by Sally Collins, it will explain in nauseating detail the how/what/where things go wrong, and how to prevent it.
It's really just checking, checking, and checking at each step of the way.
Notions - post-its, 1/4" feet, block books - they're not a panacea for understanding the construction.
And then, after you understand the how/why/where, you can totally forget Sally's perfectionist ways and go make art quilts. ;-)
Try to find the book. Really. ;-)