Well, I can offer some guesses.
We have some different terms and types of feed where I live. Are corn layers pellets all corn, or half corn and half other materials? The treats sound good.
Seems to me based on reading and experience with my flock, that individual ducks have slightly different nutritional needs. A few of my ducks have had trouble getting enough calcium or using it efficiently. Some ducks are very sensitive to low niacin levels. Those are just two examples.
So If I were you, I would get some poultry vitamins with electrolytes and probiotics, and a source of free choice calcium (crushed oyster shell, some use clean ground egg shells) and try to boost the levels of major nutrients a layer would need.
Also, low grade infections can affect laying. Extreme cold, parasites, all take a toll on a duck's health.
I found my runners did not do well at all in temperatures below freezing. Once I got them into a shelter that stayed above 40F, they started laying again. I had thought it was the short daylight, but it is a number of factors.
From what I have read here on the forum, some ducks just take their sweet time before they lay. You might find a dummy egg (wooden ones are nice) and place it in a corner of the shelter with some bedding fluffed up around it to see if she will respond to that.