How much and how often are you watering?
Overwatering and high humidity can cause this.
I find rice hulls dont work nearly as well as perlite to loosen soil. That may be contributing too.
From another website…
Overwatering will literally drown your plant’s roots. Excess water will not only rinse most of the beneficial microbes from the medium, a sodden substrate can also become colonised by algae and nasty fungi. Persistent overwatering invariably invites the parasitic Pythium, better known as root rot. Cannabis plants with droopy, claw-like leaves could be trying to tell you they are waterlogged.