Hey, a couple of pics would be helpful mate. Droopy leaves can be caused by a number of reasons one of the most common is overwatering and it sounds like that may be your problem, you say you water when the top layer of soil is dry around the edges, the top layer will dry pretty quickly you should stick your finger a couple inches in the soil and when that feels dry then you can water, another method of determining when to water is based on the weight of the pot. An important aspect of watering is having the plant in the correct sized pot for it's size, it'll be difficult to water correctlly a small plant that is in a big pot, if the roots are only using the top 1/3 of the pot and you water untill you get water running out of the bottom it's gonna be impossible for the plant to drink all the water. Start of with small pots and repot them when the roots fill the pot, (at least 2 or 3 times throughout your grow)

Good luck!