There are a few reasons why mandazis become hard: 1. Overmixing the dough - mixing over an extended period of time results in extra gluten development, which then results in a gummy, chewy or hard dough. 2. Frying at a too low temperature - the temperature should lie between 350°F (176°C) and 360°F (182°C). Frying at a too low temperature will make the crust hard and the mandazi greasy. 3. Using cold water or not using enough water when kneading - make sure to use enough water when kneading, to ensure that the dough stays pliable. You should also use warm water. 4. Not storing properly - finally, not storing the cooked mandazi properly will make them hard. You should always store them while still warm (not hot) in an airtight container or paper bag.