Is the manure you buy contaminated?
At Garden Organic‘s AGM at Ryton we were told of the effects of using Clopyralid, a persistent weedkiller that is often used to remove weeds from lawns. (Verdone is one of the brands containing it.) This form of weedkiller does not merely kill the weed; it persists in the plant, is taken up by grazing animals, and persists in their manure.
If Clopyralid is present in the manure you use to grow vegetables, they tend to wilt in the middle. So if your vegetables are wilting and you have applied manure, it is a sign of contamination and may be due to the manure you have used.