Why are my girls not laying?

I have 11 hens of various sorts - some ex batteries, some fancy things and some bantams, and I have a cockerel who seems to keep them all calm and happy. They've been with us for a couple of years now - we add every now and then, so they're all ages from 1 pullet to a couple of venerable 4 year olds. This summer their laying has been all but non-existent and very sporadic. We're lucky if we get 2 eggs per day, and one of those is generally a bantam, which actually seem to be doing better than the big girls, even though they're not known for it (Pekins).

Anyone any ideas why they might have gone so slow - some of them will lay one or two then nothing for a week - is that normal?

Even the ex-batteries (3 of them), which were all laying ddaily before we got them, are barely laying anything.

They are in a huge run with loads of grass, they get layers pellets and some corn most afternoons.

Could it be the heat? Over-feeding - would that do it? Help....

Oh, I don't feel so bad then, if other people are having problems too, thank you.