If she is lethargic and her feathers are poor (though that is an age babies molt into their first adult feathers in all sorts of weird - and sometimes bald - ways), you might look into the possibility of an external parasite. It's not common, but they can pick up bird lice (not transferable to people) from wild birds that pass through the yard, for instance. I'm not an expert on treating that, but look into the possibility, external parasites and really run a duck down and, from what I read, they aren't all that hard to treat.