If you feed them anything other than their starter they need chick grit to help them digest the food. Since they don't have teeth to chew their gizzard does the work but it has to have grit to work.
How much coconut oil are you giving this duckling? Does it seem to be helping?
Maybe and this is just a guess but there may have been a blockage inside similar to pasty butt outside and your massages and the oil is breaking it loose?
Sweet baby pic.