What do the flowers that "drop/fall off" look like? Imafan has a good point. Male pumpkin blossoms only open for one day and then shrivel up and fall off, and female blossoms shrivel up after one day and either yellow and drop or grow the fruits. Yes, definitely check at a slightly different time of the day. Sometimes the male blossoms will be open earlier than the female blossoms, and female blossoms will unfold somewhat later so you will need to catch them when they are both open. Typically, they won't open until daylight is bright on them.
Make sure these plants are getting plenty of water and nutrients and sun too, though my pumpkins and squash just crawled out (or up) to where it was sunny.
Finally, check for sucking insects on the flower stems -- sometimes they literally suck the life out of the flowerbuds. If there are any ragged holes in the flowerbuds, it could be slugs.
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