I had similar issues with my first quilts (ahem, more than just my first ones), and it was usually due to not having an accurate quarter inch seam. Since I *thought* I was sewing with an accurate seam, it took me a long while to figure things out. Even if you are just a hair off, by the time you sew many blocks, those hairs will add up, believe me.
Don't get discouraged, you will only get better at this!