It is not uncommon in this pattern for the center of the star to sometimes "volcano" a bit. I can't tell from your photo if this is what you mean when you say it puckers. If it is a bit of a volcano, this usually quilts out. Did with mine anyway. But when I did my lone star quilt I did also have to unsew the seams that meet in the middle and resew them, I think twice. Here is a link to a post of mine.
http://www.quiltingboard.com/t-22341-1.htm