Sorry this is happening.
Are you sure there is nothing in the pond that can cause an injury?
As stated before, the infection could be internal and that is difficult to treat without injectable antibiotics. Even then it might not be successful.
If you can set up a hospital tank you could treat that water with a broad spectrum antibiotic, but don't put that medication in the pond. or you could use the topical medication you used before. Here is an article about antibiotics that can be used on fish:
Bacterial infections are a headache for any aquarist and finding the right ones to use is stressful. This article helps you make wise choices as to what medications work best.
aquariumstoredepot.com
But, as Mmathis said, it might be time to let this one go.