Supplement do not always work, but most of the time their lack of is the culprit. I give all my birds a supplement once per week. Here is the product I like:
https://www.gqfmfg.com/store/comersus_viewItem.asp?idProduct=231
It contains vitamins, minerals, electrolytes, and probiotics. It also...
Another good point. Sometimes we cannot see the tiny little things. Mystical mom is a good duck owner, she will do anything for them. Even it it cost money taking them to the vet. I hope it is something easy and her girl is up and healthy in no time.
Is she laying eggs? That may cause a drop in her vitamins levels, but it would usually not show as an occasional drop in the leg movement. Not every duck is the same, but some are more sensitive to this. Being large doesn't help much either. I'm sure you have her on vitamins right now, since...