This is one of the things that all growers struggle with when a pumpkin isn't growing as fast as they like. There are a number of factors that can contribute to slow growth. Some can be fixed and others cannot. Genetic factors of course can't be fixed. Some pumpkins just want to grow slow. The smallest pumpkin I ever grew was right next to the pumpkin plant that grew my biggest pumpkin ever and both plants got the same amount of water, fertilizer, etc.
If you don't want to go to the expense of doing a tissue test to find out what the plant has and needs, then trying to give the plants a little more fertilizer isn't a bad idea. These plants are heavy feeders and they can "bonk," because although there might be a lot of nutrients in the soil sometimes it is locked up or just out of the reach of the roots.
Doing small, divided doses of additional fertilizer may not be a bad idea, but spoon feed and see how the plant reacts. Be patient with it. Over doing it, even slightly, can sometimes have a negative effect on the growth of the pumpkin. Doing it this way will probably give you better results.
To figure out what the plant needs take a look at the color and growth of the plant. Potassium can help push the pumpkin along, but low nitrogen or other nutrients can also limit pumpkin growth. Slow and steady wins the race in pumpkin growing. So look at everything that could be a limiting factor and do small tests to see what yields you the best results.