Are they outside or in a greenhouse? I have lost all of mine in the greenhouse due to red spider mite and have put in new plants. Or maybe you are not feeding them weekly or under watering them?

I have the same problem in our small greenhouse which hasn't any shade but in our larger greenhouse I have shade and the Que's are doing fine, one way I shade the plant's it to put up a sun umbrella to keep the hot sun off them, worked for me. :o))

If they are in containers, double check your watering and feeding schedules. Cukes in containers need massive amounts of water, and feeding like clockwork, with a 3-1-2 proportioned fertilizer, such as 18-6-12 or the like.