Boba is made up of tapioca starch. And as we all know starch thickens on cooling. So, after the boba cools down, it is bound to become thick and hard. To prevent the boba from getting hard, try to keep it warm. In boba shops they keep the brown sugar boba dunked in the warm brown sugar syrup. Usually, they are placed on an electric hot plate that keeps the boba warm and prevents it from hardening.