If you have poultry vitamins you can add some to her drinking water. And just make sure she keeps up with coming out to eat drink and of course that nasty broody poop.
Your a good duck mom to be concerned.
I have Runners who will be 7 yrs old in April and have never shown an interest in brooding...
I'm sorry this hatch didn't work out. letting her sit again has some risks though. Make sure she comes out every day and eats, drinks and swims if the weather cooperates on the swimming, she keeps the humidity steady with swimming, she can also get herself wet enough by splashing in a deep bowl...
Oh my gosh makes me think of some my hens[chickens] who would be in a nest box brooding and another hen would come in make the broody leave then the broody would come back and climb into another nest box to wait and for some reason forget to get back on her eggs. I would find her still off and...