Hard to answer your question very easily with the little information you are giving. Would like to know: what kind of hydroponics are you doing, what is the nutrient strength and how often do you change your solution, how do you check the level of nutrients (EC, or PPM), and what level are you using, what kind of lighting are you using, etc.

answers to those questions would be prudent.
my first thoughts are drowning and too high a temperature.
Is the drooping continuous or only at certain times?

Kind of hydroponics: not quite sure what you mean, I'm growing lemon basil plants with clay pellets in a basket with a nutrient solution below
Nutrient strength?
Solution changed once a week
Check level of nutrients: is this the pH?
Grown outdoors
Sorry, this isn't a very detailed project. Just a simple growing project.

I have four plants growing side-by-side and all but one are doing great. The last one (the one not doing well) has mycorrhizal fungi added to the nutrient solution and is the one with the very droopy leaves.

The drooping is continuous but I have noticed that the bottom leaves are more droopy than the top leaves