If she is underweight then you need to boost her protein along with cutting out the tomatoes and corn. Treats can be fine in healthy ducks when they make up no more than 10% of their diet, but in ducks that are having health issues and underweight it’s usually best to cut out treats until you get them back to normal. When you say manna pro duck do you mean the 16% protein layer? Also If not already she needs access to oyster shells in the side. If she has been having laying issues at times I would put her on calcium gluconate 1ml daily for a few days to week.