When you say fertilization process do you mean fertilization of the flowers or of the soil? High nitrogen fertilizers can cause excessive plant growth at the expense of actual fruit development. Or is it possible that you don't have many pollinators around?

In addition to what wild forager said how long ago did you plant the seeds? Also do you think it is possible that with all those vines you can't see all the pumpkins that are growing? I have some small pumpkins (~10 lbs.) that only take a month from flower to ripe pumpkin so you might still have time.