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Pumpkin growing is an art.

Pumpkin growing is an art. Some people want more (smaller) pumpkins; others want to grow one giant pumpkin. It's probably late in the game to control the growth, but if you wanted fewer, larger pumpkins, you would let a few pumpkins fruit and then pinch off any new flowers are they bloomed. You can increase pumpkin yield by pinching the tips out of main vines when they reach about 2 feet long. 

When pumpkins do not fruit well, it's because of pollination; both male and female flowers need to open at the same time and you need to have bees who will pollinate. As long as new blossoms grow and get pollinated, and your conditions are amenable, new fruit will form.