There are several factors that can contribute to poor fruit development on pumpkins: poor pollination, high heat stress, conditions that are too dry/too wet, high nitrogen fertilization, etc.

Similar to other members of the squash family, pumpkins produce male flowers for 1 to 2 weeks before female flowers appear. This is a normal growth habit and varies with cultivars. Poor fruit-set is common during rainy weather when bees are inactive. High temperatures (day temps in the 90s or night temperatures in the high 70s) will cause flowers to abort. This article from the University of Delaware goes into more detail about possible causes for poor fruiting in pumpkins.

https://sites.udel.edu/weeklycropupdate/?p=13779

Look to see if you have any female flowers forming. The females have an ovary/small fruit at the base. You can try hand-pollinating the flowers. Side-dress with fertilizer when you see fruits begin to form and keep up with watering to maintain even soil moisture. https://extension.umd.edu/resource/pumpkins-and-giant-pumpkins

Christa