If your center strip is accurately cut at 1 1/2" and after it is sewn it is less than 1", then you are not sewing an accurate 1/4" seam. It can be a VERY, VERY small bit off the cut and a VERY, VERY small bit off the sewing and the whole thing will be off. Most of us have gone thru this. The way to stop it, is practice, practice, practice. Perhaps the ruler you are using has a fat marked line so it is causing you to cut a couple threads short. Double check with another ruler. Perhaps the marking on your presser foot is a thread or so off the 1/4", check it on several places on the seam. You will learn where the true markings are on your equipment and then you shouldn't have any more problems. If your block is only off a tad (and until you get more experience), try cutting the side strips a bit bigger (1/4") and trim down the finished block to the correct size. The tiny bit bigger the edges will be will not be noticeable. Do not be hard on yourself - you are making a quilt! With each one, you will get more confident and your skills will improve.