No roots

Our plants shot up and seemed to be doing well. They r a little tall and stringy, but not too bad. But I just started to transplant and noticed we have NO roots. I'm talking a single root straight down.

We started by soaking our seeds, putting them in a napkin and them transferring to a solo cup. We r using mushroom compost. Is that too dense for the roots to spread?

What should we do?
Thanks!

you can mix it with some compost, perlite and worm castings and a dry fertilizer like rainbow mix grow and bloom mixed in would help good luck.