@YodaBat - Admit it: You're not trying to fix their bumblefoot, you are pre-seasoning your ducks!Can we rule out sepsis?
It has been contested on this board but these are our babies and cash. This is how I treat early stage bumbleds foot. If it does not cure, it helps at least.
May your Duck be well soon.

Sepsis was my first thought too! @Cayuga Lover - Your duck has chronic bumblefoot you wrote. Does she have permanent bumbles on her foot or do new bumbles spring up after the old ones have healed off? Has her LEFT foot/leg ever felt hot to the touch (hotter than the right one)?
It could be, that she has a chronic infection in her body and her immune system has been weakened over time. Now she got the antibiotics that killed a lot of those bacteria, but left some resistant onse over and her immune system was just too weak to kill those off.
If a vet is not an option - to rule out any kind of injury - i would give her high doses of FishMox (Amoxicillin) for the next two weeks, just to make sure that any lurking infection is suppressed. I would also mix Vitamin C into her drinking water, 1 teaspoon per gallon give her immune system a kick in the rump. Yes i know most of it will be wasted, but that stuff is dirt cheap.