Could be a few things, but since your not seeing it with the others that are receiving the same factors then maybe its hungrier than the others. I don't grow soil so I'm not familiar with how you'd measure but I run dwc and I would be measuring the EC/PPM/orwhateveracronym of the feeding water and then measure it again after the plant as fed for a few days. That would then tell me how much nutes to supply.
Unfortunately drooping leaves point to a lot of things so you could really chase your tale with it, I had this issue when I first started growing, I ended up buying a EC/PPM meter and then realized I was starving my poor plant... Best of luck figuring your problem out