My pumpkin plants started to have yellow spots on their leaves.

It's still very early for all the cucurbita, so it could be they have been affected by lack of water, too much sun early on etc. I've got several courgette and melon plants which are similarly affected. Usually this happens if they are planted in poor, dry soil. Always plant them in rich, manured soil, even a bucketful of good garden compost. If they are really shrivelled up then sow again, late ones can catch up anyway, but if they still have green growing points and a few good leaves, they might get going now the weather is warm, if they are well watered and fed.

My pumpkin plants have lots of flowers but no pumkins, what is the problem