Wet feather can be set on by several factors including unsanitary grounds, lack of bathing water, ectoparasites, malnutrition, and seldom a clogged/infected oil gland. Most frequently, when narrowed down, the problem overall relates to a lack of proper bathing water, with the addition of muddy/unsanitary grounds. If a duck is not allowed to bathe in deep clean water often, mud, debris begins to accumulate on the feathers, in addition, the stimulation of distributing oils across their feathers decreases when they don't have access to water.
Bathing in dawn soap should only be a one-time thing, as it is not discriminate and will wash the natural oil off her feathers as well. I would suggest you examine your setup, and her, and attempt to determine what might be a predisposing factor to this. A "mix of cracked corn" is by no means nutritionally correct and could potentially be exacerbating her problem. A feed formulated for waterfowl such as Mazuri, or an all flock feed such as Purina, or Nutrena would be a much better choice. Try to ensure she gets in clean deep water at least once a day, and consider keeping a mister outside where you can gently mist her feathers to stimulate preening.
