Sometimes softshells that small will need more authentic food to get started. Try thawed bloodworms (look like little red mosquito larvae and can be bought frozen at many pet shops that sell fish) or small earthworms/red wigglers. Calcium powder doesn't work in the water. Those pellets will provide enough calcium, anyway.

Make sure the little thing has sand to bury in. Make sure of that. MAKE SURE OF THAT. Keep the water warm. 78 to 82F will work its metabolism much better than normal room temps.