Just be careful how much you starve them. My NTT's would only eat live for the first 5 or so months I had them. Feeding them cut up earth worms may help to transition them to pellets, they slightly wiggle but for the most part just sink.
My AT's will eat anything that falls into the tank, and I think that helped train my NTTs to go for pellets.
Or you could try getting a powerhead or a strong current, so when you drop a pellet in, it shoots across the tank for your NTT to chase. The first pellets my NTTs ate were only fast moving pellets. Now they eat them just as they sink.