I have a beautiful Pumpkin growing on about an 8 foot vine, but every female blossom that has grown after this pumpkin on the vine falls off without hardly getting to 1 inch and the flower, of course, does not open. I know it’s late. I’m here in a zone 4 in Montana and we had a very long winter and a late spring start. I love winter squash.
I would say this is a pollination issue. I would try hand pollinating those female blossoms and monitor for improvement. Here is an article that will guide you on the process: