you probably have root rot, some Mycorrhizae would definitely help! preferably the kind that promotes healthy roots like
Great White premium mycorrhizae. idk what exactly your medium is made of but you may want to look in to a potting mix that has coco coir in it because its more aerated and has better drainage. typically if you dont see much run off from your plants and you know that you watered it enough for it to have more run off than what came out, you are looking at poor drainage and your roots are probably staying to wet too long which will cause root rot. Applying Mycorrhizae like the one i suggested to the top soil before you water or even just add it straight to your water will reverse the root rot but you're going to need to repot that plant ASAP before you apply the Mycorrhizae. I'd say buy a good potting mix that has bat guano or worm castings along with coco and a few other ingredients and when you go to transplant break up the root ball and sprinkle the mycorrhizae on the roots as much as you can and then plant & water. it may take some time to bounce back but as long as you apply the mycos and transplant it into some better draining soil/medium you should be alright! I added a link to the mycos i recommend.
https://www.amazon.com/Great-White-...prefix=great+white,aps,257&sr=8-1-spons&psc=1