I learned (after a long time with this problem) that when you prewash your fabrics, NEVER use a dryer sheet. I don't know if liquid fabric softener will do the same thing or not, since I only use dryer sheets. But the chemicals in the dryer sheets will not allow iron on products to adhere to your fabrics. And it is only in certain spots. I used to have bubbles of fabric that would not adhere. I found a post on the internet that suggested this and so I made it a rule to never use a dryer sheet when prewashing my sewing fabrics. You never know which fabrics that you might use with iron on interfacing. I haven't had any more problems since doing that. Whenever I teach a beginner quilting class or paper piecing class, I always tell them this in hopes that they will start off with good habits. I hope that this helps you.
Pat