I would check with a discus forum, such as SimplyDiscus
, or with the North American Discus group, forget if it's a society or what, but you can google the words 'discus forum' and it will show up.
Obviously there's a problem but I think you need long time discus keepers to help you, because they are not the average fish by any means. Don't buy any more until you get some help, if it turns out to be some organism in the tank, there is no point exposing more fish to it.
Wish you luck, and condolences on the losses. They are magnificent fishes.