If you’ve waited the recommended 72h, it is usually safe to assume that the grout is fully dried, unless you live in a humid place, like we talked about. If you want to double check, all you have to do is sprinkle a few drops of water on the grout line and watch what it does. If it beads up, then the grout is dry and good to go. If it is absorbed by the grout, then unfortunately you need to wait longer and reapply the sealer immediately.