Pumpkin woes

I am growing pumpkins at home and they keep rotting. I have tried putting them on tiles, and the photos below show some that are growing across the roof of our children's playhouse but some of those have rotted too. I have two growing well, one is on the other side of the playhouse roof, and the other is growing in mid-air, supported by a grape vine. Where have I gone wrong?

Thank you! My courgettes have been suffering a similar fate so now I know. Need to find the right flowers now to get some pollinating going. Hope it is not too late in the season!