When cutting your fabric use your ruler to measure not your cutting mat. Also, when measuring your blocks use a ruler from the same manufacturer as the ruler you used to cut the fabric. Measurements between manufacturers can vary as much as 1/8th of an inch.
Double check your 1/4 inch seam. I suggest stitching a 1/4 inch seam and measure it with your ruler to see if it is a true 1/4 inch. Does your 1/4 inch foot have a guide on it? If not you may be getting off mark just a bit.
PRESS your block, do not iron. Press is to lift up and down. If you are ironing the block, make sure you are not pressing down on the iron so much as to distort the block. You can stretch fabric as much as 1/2 inch if you're not careful. For pressing seams to one side I use the small Clover iron, then when my block is complete I PRESS it under the large iron.
hope this helps.