I just did a broken star pattern and ended up with the same problem you had. It looked terrible! As others have said the reason is the bias, 1/4 seam, etc. There is hope though. I blocked it (which is something often done after washing a completed quilt). What I did was wet the quilt in the bath tub and ran it through the spin cycle on my washing machine to get out excess water. Then I used a braided rug and stretched the top into shape and pinned (straight pins) to the braided rug. If you pin it in the correct shape when it is wet it will keep that shape when it is dry. I just finished this quilt (I'll try to post pictures tomorrow), but after I had all of the quilting completed and then washed it, the quilt kept the correct shape without all the puckering. Hope this helps! Let me know if it works for you or if something I wrote needs more explanation.