I know this suggestion sounds like it is out of left field. Did you measure your piece before and after pressing? I wonder if your fabric is shrinking a little due to the heat. Are you using steam as well?
I have never used a kit per se, but I would think fabric is fabric. I have had some shrink after washing, drying and pressing. It is one of the reasons I prewash (besides the dye running issues) religiously. I know I can always add sizing back into the fabric but it is really hard to add fabric once its been cut.