It is likely that your baby pumpkins were not pollinated. The male flowers usually start to open a week or so before the females. You can tell these because they have a tiny pumpkin at the base of the flower. In order for the females to be pollinated and produce fruits, you need to take pollen from the males and rub it on the inside of the female flowers. Pollen will get onto the stigma and the flowers will be pollinated. Do this on all the female flowers as they open. Bees usually carry out this little operation but there is a shortage of bees and maybe they didn't find your pumpkin patch. Keep watering and caring for the vines. Good luck with this year's crop.