As long as the other ducklings aren't picking on it, and it is acting normal it's ok. Continue on it's high protein feed. If they're older than 2 weeks you can throw in some more protein. I know it sounds bad, but I buy minnows and put them in their swimming pool. That keeps them active and a little extra protein. Frozen sweet peas are also good for protein that I toss into their pool with the minnows. If you don't see a pick up in a few weeks, it might be worth an avian vet