This has been a common question this year for not only pumpkins, but also other members of the cucurbit family as well such as squash, cucumbers and watermelons. And the key to getting all of these plants to set fruit is pollination.
Cucurbits like pumpkins produce 2 kinds of flowers, one that produces pollen (male) and one that produces fruit (female). It is necessary for the male flower to pollinate the female for pumpkins to develop.
You can distinguish the female from the male in that the female flowers have a small fruit, or swelling, at the base near the stem.
Male flowers usually appear first, with the female following. So there is no reason to panic if, initially, the flowers don't set fruit.
However, later in the season if you see both male and female flowers and still don't have any pumpkins developing the problem probably lies in lack of pollination.
There are several reasons why pollination does not occur. Environmental conditions such as unusually hot or cool temperatures, excessive moisture or low light will reduce successful pollination.
Honeybees and other pollinating insects are invaluable helpers in the pollination process. If you use floating row covers, harsh insecticides or even organic insect repellents you reduce the ability of these garden helpers to get their job done.
Now there is not much that you can do about the weather, but you can take steps to encourage pollinating insects to visit your pumpkin patch.
If you are using floating rows covers to protect your plants from harmful insects, remove them when female flowers begin to appear. Repellents such as garlic spray should be applied a few days before the flowers open. And only spot use insecticides for problem areas.
You can also hand pollinate female flowers. Using a small paintbrush or cotton swab, collect the pollen from a male flower and transfer it to the top center of a female flower.