I have had that happen to me, let it mature. keep the water clean and maybe some ferts to help your plants dig in.
To add onto this, most if not all plants can come back from being little to nothing, so as long as you see even the slightest hint of new life I would stay confident that things will grow back with proper care.What your experiencing here could just be the plants getting used to the new substrate, i would assume the same, if not a similar event happened to you when you got the plants, just look for new growth on the plants to tell if their healthy or not, if no new growth appears then your plants are infact dying. good luck!