First, it could be the cutting. Are you using your rotary cutter straight? Or do you lean a bit? This can make a difference. A couple threads per cut add up quickly in a block. Second, your stitching. Practice your 1/4" A LOT. You will be happy with the results. But, if you do have a block off a bit, a good thing to remember is 'we are bigger on the bottom'. This means to put the bigger piece on the bottom and the presser foot will help ease in a small amount.
Third (and most important), just keep quilting!!!! The more you quilt, the better you will be. Small errors are actually new design elements. Chances are that no one will notice but you. Quilters can be so hard on themselves, just have fun and keep quilting.