I believe you are saying that the tuber sprouted in the ground but is not growing because there are no feeder roots. Some tubers are slow to send out feeder roots and an old timer said to cut off about 1/2 inch off the end of the tuber and re-plant. That stimulates the plant to grow roots. Alternatively, there could be an insect that is eating the roots and you would have to treat the soil with some insecticide.