If the problem is too-fresh, too-concentrated chicken manure, and IF your bed has good drainage, it might help to overwater heavily a few times.
Try to water so heavily that some water flows right THROUGH the bed and out the bottom, carrying away some of the excess salt and nutrients from the manure. (Like flushing a flower pot.) This is more likely to work if the bed is rasied and you have very good drainage under it, or sandy soil.
Can you post some photos? If only some plants are suffering, maybe the problem is something else.
If the few plants that are suffering are just more sensitive than the others, you could yank those out and plant something else in their place. Either find some veggies that are more tolerant of over-fertilization, or dig out some of the enriched soil in that spot and mix what remains with less fertile soil. Spread the "hot stuff" thinly on top of some infertile patch elsewhere.
