It might be that the seams used to sew the pieces together are wider than 1/4". This could easily cause the entire block to have the stars right to the edge. I don't know of any star pattern that doesn't allow for the 1/4" on the edges, so it's possible this is the issue. I do know when I first started quilting, I was using too large of a seam allowance - thus my pieces didn't fit right and caused many problems (including this one!). You may consider double checking that, and possibly even using a scant 1/4" instead of a full 1/4" (some machines take a bit too large of a bite at the 1/4" mark, so please measure for accuracy).
I hope this helps :)