Just be patient. The plants are growing like crazy but it is all underground. They are at a stage where the plants seem to rest while the roots grow. Don't worry very soon they will start vining and completely taking over your garden.

Starting the plant inside and transplanting it was probably the problem.

Pumpkins, and cucurbits in general, hate to have their roots disturbed and suffer badly from transplant shock.

If they've been there for a couple weeks and have lived this long they'll probably take off and start growing any time now.

The best addition to your sandy soil is compost. You can also put a wide ring of compost around the plant, but away from the stems themselves. Pumpkins are "heavy feeders" and like to be fed with compost that way several times during the season.

I can't tell from the picture, is your sand the beach sand type (white or yellow with rounded grains all about the same size), or the glacial till type (a grubby-looking grey or brown with sharp grains of different sizes that seem to be made of different minerals)?

Weirdtrev is probably correct, and it'll snap out of it. Though, I really prefer to plant seed directly in the garden. It generally does better. In most of NJ you still have at least a week or two more that you can still direct seed and harvest most pumpkins before frost. So, if you have any doubt, you might plant a few more seeds, directly into the garden soil.

George
Tahlequah, OK (formerly of NJ)