One thing that I don't think anyone has mentioned---after you have completed a seam, check to be sure the bottom layer of fabric has not slipped. If the raw edges of the fabric are not aligned with each other, all the way across the seam, there is no way the blocks will be accurate. Takes a bit more time but a pin, perpendicular to the edges of the fabric, at each end and one or two in the middle might help. Just slip the pin out right before it goes under the presser foot. Also, when you press, be sure you haven't accidentally ironed in little "pleats" parallel to the seam line. I set the seams by ironing the 2 layers against each other, then press to one side from the back and THEN turn patch over and press again from the right side. Use the side of the iron and gently push against the roll of the seam to flatten out any hidden (or obvious) tucks or pleats.
Happy piecing/quilting! Don't give up!