Always make sure to dry out your goggles in the open at room temperature after each day of use. If it was especially wet that day, we recommend that you remove the lenses (see How do I replace my lens?) and have them sit and dry next to your frames. The goal is to completely dry every piece of the goggles.
If fogging persists the anti-fog may have been removed by improper handling of the inner lens, or use in excess moisture. Buying a replacement lens would be the final solution.