Holy smokes you weren’t kidding about drooping! Hard to tell exactly but given the size of it and that it was recently healthy, and the others still are, I would suggest something root related. Perhaps bugs if it isn’t a drowned plant situation. You also mentioned working stuff into the soil on a weekly basis - I would be really cautious about disturbing the roots. It’s possible you may have damaged some of the root system. Maybe try making your nutes into a tea rather than working stuff into the soil. There might also be some type of environmental contaminant right where the one plant is - maybe near a gutter or something so it is getting chemical runoff from your shingles or something like that. I am also just guessing here so it could be any of these or none.
A final long shot suggestion, not sure of the bugs you have in your area but we have squash bugs here that basically lay eggs in the vines of squash plants and as they grow they cut off the water supply for everything farther along the plant and it goes wilty and looks under watered and dies no matter how much you water. I don’t think they normally do cannabis plants but given cannabis often has hollow stems I think it is possible, especially if some of of the lower leaves look ok. If that sounds possible look for frass on the stem. You can sometimes pull out the larvae and rescue the plant but not often before it is too late. Long shot but possible.
