Sweet potato seedlings can germinate smoothly as long as the water content is sufficient and the temperature is suitable (generally sweet potato seedlings will be covered with shed or film). Fertilizing sweet potato seedlings is to provide enough nutrients for them to gain weight.
Generally speaking, sweet potato seedlings are fattened by farmers, and chemical fertilizers are rarely used.
Try using farmyard manure instead of chemical fertilizer. If it does not germinate again, it is estimated that the most likely reason is that the temperature is too high or too low, which leads to its non-germination.