Hi Smug,

Sometimes this condition is caused from roots dieing from lack of oxygen due to the soil being too moist. If the whole leaf wilts this is normally underwatering. There are also a couple of other thing which can cause this the most common one is a virus (no cure). You should get your self a cheap humidity gauge if the grow room is too wet then the plants can't expel water and this can also cause them to wilt. If you find that your soil is almost always wet then this is probably the problem.

Good luck with them and I hope it works out.

Herbus