Are you pressing your seams open? I know it's OK to do that, but when I do I can't feel the seams nesting together and they will slip. Press your seams one direction on the first row and the opposite direction on the next row. When you put the seams together you will feel the perfect fit between your fingertips. Then if there is a slight mismatch you can ease the difference on each block.
The perfect 1/4" seam is not as critical as some suggest. What is critical is that EVERY seam measures the same, whatever that measurement is.