I don't know much about pumpkins, have never had room to grow them. But in general, blossom drop (as in tomatoes and peppers) is a sign of stress. The plant drops the blossoms/ potential fruit to focus on survival.
Kinds of stress that can lead to blossom drop include:
Soil temperatures and/or night time temperatures too cool for them, below 60 deg F.
Cucurbits are warm weather crops, but still it can get too hot for them. Once daytime temps get well into the 90's and/or night temps well into the 70's, it can start to stress them, especially if it is also dry.
A sudden shift from hot spell to cool temperatures.
Low soil moisture as a result of drought or lack of irrigation or too much wet-dry cycles. Use compost in the soil and mulch to help maintain consistent moisture.
Too much soil moisture as a result of rain. If summer rain is frequent, plant in well-draining raised beds or grow plants on mounds.
Too much nitrogen in the soil. Excess nitrogen can cause rapid, succulent growth and disrupt a plantâ€™s metabolism. Avoid high nitrogen soil additives such as bloodmeal and fresh manures. Use low nitrogen fertilizers such a weak compost tea or side-dress plants with aged compost, a balanced soil amendment.
OR too little nitrogen and other nutrients.
Planting at too high of a density (closer spacings), especially with high nitrogen, can cause excessive foliage and increased shading that will limit early fruit sets.
What do you think; any of this ringing any bells?