This could be the problem. Potting mixes aren’t usually designed as a seed raising mix and often can be acidic (from carbonic acids through composting process) or not adequately composted causing severe nitrogen deficiency. Both along with other characteristics increases the mortality of seedlings soon after germination.
A light sandy loam topsoil, or that in one’s vege patch is usually sufficient media for seedling growing.
If you want an excellent text on potting mixes, growing media, see this other post:
