it is 1 of 3 things,
1.your cutting is off, (sometimes the rulers slips a little
2.or your seam allowance ,
3.or your pressing is a bit off.
when you first start sewing..i find that with beginners, the fabric is tight up against the 1/4" seam black bar on the foot, then as then sew, it,(the fabric), gets farther away from the bar..so that when you finish the piece, the seam allowance is right at the start and wide at the end.
clear as mud? lol
how to sew an accurate seam allowance:
http://quilting.about.com/od/machine..._allowance.htm
How to press your blocks:
http://quilting.about.com/od/pressin...r_Quilters.htm
How to rotary cut your fabric:
http://quilting.about.com/od/rotaryc...our_Fabric.htm
measure the seams after you sew,before pressing..and let us know how it measures up. a picture would be more helpful.
and when your ready to do a nine patch,we will be here to help you! easy peasy!