Cracking Stubborn Shells Manually
After five or six days in the towel with no taproot, a seed can be rescued by gently pressing the seam with fingernails until it clicks open. Too much force crushes the embryo, so it is a last resort for old or thick shelled stock rather than a routine step. The technique, including how to hold the seed and what a successful crack looks like, is shown in photos on https://kushindicaseeds.com alongside the failure cases. Afterwards the seed goes back in a damp towel for another day or two.