It's a very common problem with growing seedlings, especially indoors. I've only seen it rarely since I use sterile potting soil and I did this years ago before I was using Promix. The seedlings die off by dieing at the ground level. I probably had the seedlings way too overcrowded since I had a very good germination rate and not enough air circulation.
I still ended up with a lot of delphiniums. I think I had the best germination with collected seed that I stored in the fridge over winter.
