Make sure you use inks and paper that are specifically for sublimation. It sounds silly, but I encounter numerous problems that can be solved by simply using the proper equipment and materials.
The heat temperature should be at 400 F, and you should allow enough time for the sublimation process. The most common mistake that sublimators make when it comes to this problem is that they press with the coated side (also known as the face) down. Always press with the face of the product up.