The seeds germinated but the roots did not develop

This has happened to me a few times too and think like Mouse said, it could have gotten a pathogen on it and as it pops it gets attached and rots. I have heard of people hydrogen peroxide to sterilise the seeds before germinating to stop that from happening, but never tried it myself. That seed is unfortunately gone, but you can try H2O2 on the others if you have any more. Also keep the paper towel only slightly damp after the seed pops. Too much moisture can also cause the seedling to drown and rot.