Good cutting, as you are doing, is only half the battle.
The second half of the battle is to make sure that your seam allowance is exactly 1/4 inch. Most machines are not set to do this by default, EVEN if you are fortunate to have a 1/4 foot. Take two scrap 2" squares of fabric and sew them together. Then using a reliable ruler, not your cutting mat measurements, measure what the length of the finished unit. It should be 3.5". If your unit is smaller than this, it means your seam allowance is too large.
Some machines allow you to press a button to change the needle position. Mine has to be clicked twice to get the needle moved over enough to the right to make a 1/4" seam.
The pattern isn't too hard, because you are so conscientious and you are doing everything else right!
Good luck.