Sew 3 strips of scrap fabric 2" x 6" together. The strip set should measure exactly 5". If it doesn't, then you are not sewing a correct 1/4" seam. You can adjust accordingly. If you can move your needle position to the left or right on your machine, try doing that to get an accurate 1/4" seam. Then do your test strips over again with new scraps. A quarter inch foot helps also. Personally,I like to sew a "scant" quarter inch which means you need to sew a thread or two less than 1/4". By sewing a scant quarter it gives you a little allowance for the seam and pressing. That works well for me.
I have had trouble in the past with all my blocks in the BOM club not being the same size and rather than tearing them apart and redoing them, I have put a 1" - 1.5" border around them and then squaring them all up to the same size.