As Janedee said, if you're not using a walking foot, that could cause the problem...one layer will shift but not the other, causing the bubbles of fabric. Also, is it possible that you basted it unevenly? The block next to this one seems to be OK where you did the squares or spirals. If you are concerned that the point of the white square doesn't meet the seam, you could take the crossing seam out and restitch by hand to make the point come closer and also to take out the extra fabric. Since you only show us one part of the quilt, its hard to tell how "important" this little bubble is in the whole look of the quilt.