Trust me. As much fenbendazole you gave your birds, they would not have worms. Yet the symptoms kept reappearing. That is typical for some type of respiratory disease or something in the environment causing the symptoms. Examples could be; inhaling feed dust while eating, dust from dust bathing, pollen, pesticides, ammonia fumes from soiled bedding, improper ventilation, fungus or mold spores, smoke.
If it's some type of respiratory disease, it will show symptoms in birds when they are stressed. Birds that survive are carriers for life and will spread whatever disease it is to other birds.
Also just for your information, birds that are kept on the same soil are more susceptible getting worms than birds that free range. If their feet touch the ground, they will get worms. So, it's a good thing that you have wormed your birds. It is true that wild birds can easily introduce parasites to your soil. I've had it happen with migrating birds. The only thing you can do is worm your birds on a regular basis and fenbendazole is a very good wormer. However if you have doubts about the product, it's very easy to gather up fresh fecal samples and put them in a ziplock bag and take them to a vet and have them look under a microscope for worm eggs. It doesnt cost much at all. Then, the results will verify whether the fenbendazole was effective or non effective. Then you'll have your answer. As always, it's best to give wormers orally to each individual bird. That way you know they got properly wormed.
Putting wormers in feed or water isnt very effective at all. You dont know if all the birds ate or drank it, or if they ate or drank enough of it to be effective, and sick birds rarely eat or drink at all.
I also recommend that you thoroughly inspect your coop/pen for any of the deficiencies I mentioned regarding environmental issues which can be taken care of on your part.
