My cucumber plants aren't sending out runners?

There are two types of cucumber plants, vining and bush. If you have the bush type they don't send out vines. Although you very likely will have to provide them with support if they are heavily loaded. The easiest way is to put some stakes in the ground and loosely tie the plants to the stakes with strips of cloth to hold them up.

Cucumbers love to climb and if it is not supported it is going to tip over and run on the ground. if you are in a small space use tomato cages, if in a larger space, next time you plant give it a trellising of some sort. I use tomato lattice that actually bends around the plants...except I run the straight and support with bamboo poles on either end. They climb and I have to less bending over to harvest.