Perhaps they were not fully pollinated or maybe they were poorly located on the vine, competing for nourishment with a more developed neighbor. The harsh truth is that not every tiny pumpkin is destined to make it to the end of the season. Top Pumpkin plants are hardy and strong, but like all living things, they are vulnerable to outside forces. In the early stages, the main danger is frost. If the young plants are in the ground and the nights threaten to become very cold, protect the seedlings with inverted clay flower pots or a small cold frame, removing the covering each morning.
Strong wind is another threat to the sprawling plant.
As the season progresses, many insects and critters will visit. Almost all are friendly and many are simply on their way to another land. Even snails seem to overlook pumpkins, preferring, instead, less fuzzy fare. In different regions, pumpkins are variously plagued by gophers and moles, vine borers and beetles, aphids and mildew and other unwelcome visitors and conditions that threaten their security. When these forces find their way into your patch, they will need to be discouraged.