The fact htat you know the length of the taproot alarms me. The root is EXTREMELY fragile to touch, light, oil from your fingers and a thousand other things. U should never be able to see your taproot or u are doing something wrong imo. Once that root sees light or u touch it, it is damaged, it sounds to me like u may have damaged the root when it was very small. I use sterilized tweezers to touch my seeds, even before they have germinated. If it has leaves and has not been planted permanently u are handling it way too much.
I recently had 100% success by putting the seeds in a shotglass of tap water for 4 hrs, then damp paper towel overnightin dark/warm place. Then plant in soaked and drained root riot/rockwool the next morning. NEVER touch the seeds with ur hands and get them planted in the rockwool just deep enough that u can cover them with a little piece of rockwool (seeds length deep). Then put them in the dark in a warm place and DONT TOUCH THEM for a few days. The rockwool or whatever u use will stay moist for a couple days, but u should check on it to make sure, nothing else is required but water until they break the surface.
