What are the tank conditions, water parameters, hiding places, tank mates?
Your NTTs, have recently been scooped out of the Mehkong river, an enormous body of water with zero nitrates, many places to hide where they are invisible to all other fish. They have naturally eaten small river fish and freshwater shrimp ... alive.
Can you recreate this? If not, they may never eat.
Try some live ghost shrimp.