No insect or anything else. They are open only a few hours, then drop on their own.

Thanks Jean, I hope it was what you said. The fact is the flowers were not opened and I could see the tube where flower was cut still oozing with juices and this still happened this morning. So????? what is going on? Any other inputs?

You may have an animal, insect, or bird that likes to eat flower blossoms where you are that we have heard nothing about. If there is no insect larva to be seen then it is not that, because even after the damage is done the larva would still be there. If the garden is not fenced in tightly you might want to consider doing that, or maybe using floating row covers (which does mean hand pollinating).

Everything sounds normal.

The male and the female flowers are only open for several hours each morning.

The male flowers drop cleanly from their stems, as if clipped off. The females, though, stay on for quite some time, even if not pollinated.

Kimmsr, the pumpkin plant is inside fenced area. Unfortunately my first ever pumpkin dropped off this morning. However, there are a few female flowers coming up.:) I hope by them I can have more male flowers bloom. Do I just plug the male flower and brush with the female when do hand pollination and early in the morning? (I don't see much bees and other insects around). There was another unopened male flower bud been cut off when I checked this morning :( and there was one blooming:)

I finally came to realize this after careful inspection on the dropped flower; it rots before bloom including female flowers:(. Why???? Too much water? (lots of rains these days and weather forecast there will be lots of rain in December and January.) Looks like I could only pray hard for the god to give me some fruits!