When do protons and neutrons strongly attract?

When they're very, very close together. Protons and neutrons are not attracted to each other electromagnetically, as neutrons have no electronic charge.

Protons and electrons are, as one has a positive and one has a negative charge.

Instead, Protons and Neutrons are held together in the nuclei of atoms by the Strong Nuclear Force. The strong force (also called the color force) is the strongest of the four basic forces, but it's only important over extremely short ranges.