The only thing I can figure, is since it is just a peat tablet, there is no
nutrition for the plants.
Seed starting medium needs to retain moisture but be well drained, support
roots, and offer a bit of nutrition. I make my starting soil from sifted
forest humus, garden soil and sand. The prime problem is damping off
disease, which is a generic term for a multitude of seed and seedling
diseases. Damping off can largely be avoided by the simple, though smelly,
expedient of baking the starting soil on a cookie sheet in the oven. For
serious gardeners this is very time consuming and even homesteaders may be
forgiven for buying sterilized starting soil.
If heat and seedling soil are adequate, the primary reasons for seedling
failure is inconsistency of moisture and inadequate light. You can't start
seeds and go on vacation.