You can also use ivermectin paste. I believe speckledhen has said she uses a bb-sized bit of the horse paste wormer ivermectin (generic is fine) in the beak. One tube of ivermectin (read the active ingredient) is the same as the next, usually all treat about a 1200 lb horse.
If your vet can...
I agree with both Ivan3 and speckledhen on this. I would treat for gapeworm and then try to rule out the causes listed by Ivan3.
Here's information on Ivermectin should you choose to use it:
https://www.backyardchickens.com/forum/viewtopic.php?id=185989
It's a long post so I'm not going to...