I've done some accidental pumpkin gardening in the past (threw the Halloween pumpkins onto the compost pile behind our yard) with good success.

This year I purposefully planted the seeds from last October near (not in) our garden, around mid--April. The soil is not great but it is full sun + and they are watered regularly by our sprinkler system.

Two pumpkin plants have now each grown to about 20-30ft in length. Lots of flowers, and initial pumpkin fruits but the fruits don't grow bigger than a silver dollar before they wither up. Is it a soil issue? I can't believe it's due to lack of sun or water.

Here is a link that might be useful: