squash - why don't the flowers become fruit?

The insects will pollinate them. Sounds like you have the same problem that many do this year - all male flowers/plants. They don't produce the fruit. The female flowers/plants do. Its a case of bad luck this year. Hopefully you have a friend or neighbour who has too many squash.

But I think its a bit late in the year now to turn hermaphrodite. If they will at all. I've never had any change when there's a shortage of the other.

I had all male cuke plants 2 years running (from different seeds)  ( Not a single cuke in sight. 

I grow Winter festival squash and also courgettes; don't have problems getting male and female flowers to appear at the same time but they do seem a bit shy sexually,  hence my morning "paintbrush patrol" (the flowers close up in the afternoon) to help them along a bit (bit of a tickle if no slap!)

I know insects are supposed to do the job for you, but it is so disappointing seeing what look like young fruit rotting away due to lack of pollination it seems safer to give them a helping hand if possible.

(I also take off the flower after it wilts which helps stop blossom end rot in wet weather).