Add more aeration to your soil. You need perlite rice hulls or other types of aeration to keep the roots happier. I don't transplant until there is very good root development. Transplanting too soon can easily lead to overwatering. Cannabis thrives in a good wet/dry cycle so your soil should be able to dry in 3-4 days and you can do light waterings while the plant is small to keep the medium on the drier side to as not rot the roots. The roots will be in a constant state of decay if the medium is too moist on average
Sorry for Your little Plant. Looks like it was way to wet for her. Maybe a weak Seed as well, i have one Bonsai too in my run. 5 grow regular but one is still small and poor, even if all have same conditions. So i wish You better Luck with the next seed.