It could be any number of things, but it looks like you are over-feeding. Calcium excess, in particular, can interfere with new growth (visible in the 2nd picture), interfere with potassium uptake (tell-tale of brown leaf-spots without chlorotic yellow halos) and cause leaves to "droop", as is clearly visible in all 3 photographs.
This is a very small plant and, from the looks of those hard water deposits on the foil surrounding the plants, your water probably has a pretty high TDS content. I'm guessing you are just using plain tap water, correct? If this is the case a fertilizer that doesn't contain as much cal/mag and other trace elements would probably be better. You only need these things in trace quantities, hence the term "trace elements". While calcium isn't really a trace element and is often even considered a macronutrient, it is needed in much lower quantities than NPK and is commonly abused by new growers, along with magnesium.
Also, what is your relative humidity?
This certainly doesn't look like a lack of humidity, but in one picture I see what looks like condensation on a plastic dish in the background, so you might need to improve your ventilation. High levels of moisture can interfere with the transpiration of moisture from leaves which leads to drooping. Note that "drooping" and "wilting" are two different things. It's helpful to learn the subtle differences to help you distinguish between two similar, but different issues. I keep my humidity at 60% indoors. You can use a whole room dehumidifier outside of your actual grow chamber as long as you measure RH inside the chamber. In this case, you may need to drop the RH outside even lower, say 40% to get the inside of the chamber low enough.
Interestingly enough very high RH can also increase mineral accumulation and aggravate the symptoms of over-feeding.
Get an RH meter and TDS meter ASAP.
