One thing is the type of fabric you are using for both pieces. If they are both high quality 100% cotton, that shouldn't be causing a problem. If you are sewing with fabric that has a loose weave or is polyester or has some kind of "slippery" surface, that might be the problem.
Another thing to check is the tension on your machine. The top and bobbin threads should meet and cross as close to between the fabrics as possible. If you see that one thread is tight and you can see little loops where the other thread crosses, it's time to adjust your tension - either the top or bobbin.
It could also be that as you are sewing, one fabric or the other is stretching a little as you feed it through the machine. You might try pinning your border strips on before you sew. That should keep the fabrics lined up.
The first thing I would check are the actual measurements of the 6 1/2" block. Hopefully your blocks are more accurate than mine. Most of the time when I think my blocks are 6 1/2" (for example), the actual measurements could be anywhere from 6 1/4" to 6 3/4." Sometimes that will make a difference in puckering as borders are added, and sometimes it will be ok.
And then check to be sure that the blocks are actually square, and not a parallelogram. Often my blocks are not square when I finish sewing them, and they need to be trimmed - to get the sides even and to get them the same size all around.
Hope that helps.
Happy Quilting!
Penny Halgren
www.How-to-Quilt.com
www.TheQuiltingCoach.com
