I used jiffy pellets and they worked great for my first round of seeds. Years later I ordered another batch of seeds, got a new box of pellets, and everyone died shortly after germ. It took me a whole other batch to figure out that the pellets, apparently, had something wrong with them. As soon as I switched to rockwool I got regular results again, and since I have germed in several other mediums. I don't know what could have been wrong with that box of pellets, but they sure seemed to be the problem.
I stopped using expensive seeds and got some midi seeds from a friend. I used those until I figured out what was wrong.