I've had a couple do this to me too, in the same soil as others which have grown perfectly fine. These days I just chuck them out if it doesn't improve.. dug a few up before and the roots were totally undeveloped. It's a pain to waste a good seed..but .. I guess sometimes you might just be growing the runt of the litter. If you've ruled out all other options, it can be easier just to start afresh. That being said you've got 2 strains doing the same thing. I'd say your soil is a bit funky. You're growing fully in doors right? Would there be any way that your soil could be infested with any pests that might be attacking the roots?