sounds like the flowers are possibly not getting pollinated for some reason - as the seed are not fertile, the fruit gives up and collapses. Have you seen any bees or any other pollinating insects like hoverflies around? You could try hand pollinating (take off a male flower and dab the female flower - or use a small paintbrush or make up brush and gently transfer pollen)
Courgettes and other squashes like very fertile soil - maybe the soil needs some rejuvenating? Bit late now, but maybe for next year. You could try watering in some Epsom salts (magnesium sulfate) which could help with fruit set if that has been the problem.
Again, too late now but next year you could try growing a parthenocarpic type - these do not need pollinating insects to produce fruit, and tend to be early croppers and go on throughout the summer too (can be grown in a greenhouse though I would only suggest that for someone who has trouble growing them outdoors due to a cool wet summer) I have grown Cavilli F1 (seed sold from
Thompson & Morgan - hmm, don't remember it being that expensive, maybe I got them from somewhere else [EDIT: found them in
Tuckers Seeds] The other variety is Partenon F1 or Parthenon F1 (I think it's the same -
Dobies and
Thompson & Morgan ... they've always been expensive for what you get, but you are guaranteed courgettes