I have one duck that seemed to do a similar thing. She was fine at first (we got her three days old) but eventually (around two weeks) she would only walk a few steps then just plop! She was always laying down, or "sitting" like yours. We were about to take her to the vet after three continuous days like this, but we kept up a normal routine of bathing and would encourage her to follow us and use her legs. She finally learned to balance and walk her own way. Mine is now pretty hefty compared to my other duck, so perhaps it was her weight? But she doesn't seem overweight, just awkward. I hope your duck recovers just the same!
I attached two pictures- the black duck is our "normal duck" with perfect posture, and the gray one is the fat one who had a hard time walking. She seemed to adapt by resting her body on her upper legs and having a wider, almost pigeon-toed stance. Unfortunately she learned this on her own so I have no tips. I just want to let you know you are not alone!
