Try measuring all the squares in the row. The first and last square should measure 4.75 -- all those in between should measure 4.50. If they all measure the same, they should lock together. However, if they are off slightly, just pin. Line up the seam, stick a pin straight through the seam (look on the back to make sure the pin is coming out through the seam and not the allowance), keep that pin straight while placing a pin on either side catching the allowances, then remove the first pin leaving the two in the seam allowances. I place my pins so that the points are toward the edge. They seem less likely to get caught on my presser foot that way, and they are easy to remove with my left hand. Also, when possible feed the fabric so that the seam allowance is toward the needle rather than facing away. I find that the pinking shear cut on precut pieces are difficult to line up evenly and get exact measurements. I highly recommend Harriet Hargrave's Quilting Academy Series. She has four books out - Freshman, Sophomore, Junior and Senior. I had trouble getting my seams to lock until I followed her lessons. The first time I put seams together and didn't have to pin was exciting. Her methods aren't speedy, but they are accurate. Getting the correct measurement of your square or block is more important than an exact 1/4 inch seam. Good luck.