To obtain pumpkin fruit your plants need to produce both male and female flowers, they need to be open at the same time, and bees need to be active when the flowers are open to pollinate them. Sometimes only male flowers form. Male flowers have a long, thin stalk. Female flowers have a fruit attached. It will be very tiny when the flower starts to form. Bees may not be active if it is cold and rainy or very hot when flowers are open. A dense leaf canopy can interfere with bees finding flowers.  Your local extension office will have information about successfully growing pumpkins in your area.