Duckling not Eating

I am new to ducks and yesterday I got 3 new Cayuga ducklings. I've noticed one of the ducklings is noticeably smaller than the other two. It sleeps a ton and peeps a lot more than the other two and while it falls asleep. i've noticed that it isn't eating. It will drink water when I put it near the bowl but then it will walk back over under the lamp and sleep more. I thought it was showing symptoms of just being too cold so I lowered the lamp to hopefully make the smaller duckling warm but then the other two ducklings hid in the shadows behind the waterer and it seemed too hot. I gave them all meal worms as snacks to get used to me and the other two loved them but Tiny was uninterested. I'm not sure if Tiny is just a runt who needs some kind of special attention or if it's a temperature thing or something. If anyone can help me out or give me some tips that would be awesome. Thank you!
- Tiny has also seemed to make the movement of trying to poop but nothing has come out this has happened about 2 or 3 times that I've noticed
- The other two are almost identical to each other in personality and looks whereas Tiny is just so small and tired