my pumkin plant is producing pumkins, but when they reach the size of a golf ball they rot and drop off! do you know what I am doing wrong please

mine started doing this, I was told you need as in most cases male and female flowers to get any of your pumpkin plants to produce pumpkins
the female apparently have a bump underneath the flower I think it was
x x x

As in the answers to similar courgette problems, I think this is to do with poor pollination of the female flowers with the baby pumpkins behind them. Especially in poor weather the pollinating insects don't get round to doing a good job. You could try hand pollinating to see if this brings an improvement.
But it's normal for the small pumpkins not to develop once you have already got some large ones growing on the plant.