Let’s try this’ what type of orange oil did you use and in what solution mixture/ratio? Were all plants sprayed from the same mixed batch of solution? Do you remember if the plant with the problem was the last to be sprayed?
　
I have been looking for the pH level or orange oil but since it comes in so many different forms/concentrations for different uses what I have found varies a good bit but all are anywhere from rather acidic to very highly acidic.
　
What I was just wondering is if that plant was sprayed with a different batch of solution that might have been mixed to a slightly different ratio or sprayed last and the mixture hadn’t been evenly mixed and or there had been some settling and it received a higher dose per drop?
　
It just seems to me that the plant likely got the heck shocked out if it and you did use an acidic substance.
　
Since you sprayed the plants checking the pH level of a reservoir or of soil would not show up the what had been taken in by the plant in what to it would have been something of a distasteful foliar feeding.
　
Consider acid rain. Mostly it is said how the acid rain effects the soil, it dissolves the nutrients and helpful minerals in the soil and then washes them away before trees and other plants can use them to grow. At the same time, acid rain causes the release of substances that are toxic to trees and plants, such as aluminum, into the soil and the combination weakens the trees and that leads to them dying from disease or some other problem.
　
Well scientists also believe that when leaves are frequently bathed in this acid fog, like trees at high altitude, essential nutrients in their leaves and needles are stripped away. If acidic moisture trapped in clouds surrounding trees and plants can strip essential nutrients from leaves and needles do you think the same would likely occur if an acidic solution were sprayed onto plants?
　
It may still sound unlikely to you since you said all the plants received the same spraying/treatment and conditions but I still have to believe the cause and effect here is still the spraying even though only one plant has shown signs of problems.
　
Since all plants have received the same care, feeding and conditions if some over-amount or deficiency of some element(s) were developing it may have shown up first in one plant but likely very soon after in others and by now you would have said plant #2 is drooping too and you haven’t so I doubt it has anything to do with what happened previous to the spraying.
　
Possibly through some mishap that one plant received a heavier dose or possibly it has some slight genetic difference that makes it more sensitive or maybe even hyper-sensitive to certain things so it reacted quickly and to a large degree while the others just shrugged off the same experience/condition.
　
Some good clear pictures of both the broad type and close ups, that were of course hopefully clear, would she be nice. They may show exactly what the problem is so it might be able to be solved somewhat easily but if nothing else they may help to limit the amount of hypothesizing that will be done before a solution is found.
this sounds bad! But i talked to a VET and they said this product is better than neem oil and to follow the instructions on the label to a T.