Okay, the ointment may not be working in which case it's probably not external bacteria.  So I would stop that at least for a few days.
Please go over again:
what they are eating
what their drinking water is like
how often they get a warm bath
if you have a head washer in the brooder
what the bedding is
how warm the brooder is
if there's anything with a sharp edge on it in the brooder.
For now, I would use old towels for bedding, I would make sure duckling's getting two warm baths a day, make sure the brooder is dry and not swampy (it can be done, please search the forum for ideas), I would get a larger brooder box, I would get a thermometer to make sure the temperature is right and that there is a cooler spot in the brooder, I would get a magnifying glass and check for anything unusual on the skin.