Do the grow bags have drainage? What do you consider "a lot of sun"? What side of the house are these plants against (north, south, east, west)? What type of growing medium are they in? Are you watering with tap, well, or rain water? Were these grown from seed or are they transplants? When were they planted? Where are you located? What state?

The grow bags do have drainage holes at the bottom of the bag although I could put more holes in them if necessary. What I consider to be a lot of sun is sunshine from 9am until about 7pm perhaps not directly shining on these plants the whole time. For direct sunshine Id say they get about 3 hours afternoon sun shining straight on them.
The growing medium is basically what come in the grow bags with no added nutrients. They are Levington grow bags and I couldnt imagine that is what is causing the problem because 1) I have more cucumber plants growing in exactly the same type of bag which are growing fine and 2) the pea plant that has the same problem\symptoms are growing in a pot filled with John Innes No. 2 .
I am watering with tap water apart from when it rains obviously. Again if it was the water that was cauing the problem I would think a lot more plants would be affected. They wree planted outside about 2 weeks ago just after been hardened off for a week because they were grown from seed. I am located in southern England. I dont know if this would have anything to do with it but these plants are situated near the rubbish bin store which can attract a lot of flies at times.