If you're sure that the tank is full and the priming switch isn't producing any water pressure, then you probably have an air block in the pump. The easiest way to bleed this air out is to:

1) Loosen slightly the hose clamp next to the output of the pump (see white arrow). Don't remove it, just loosen slightly.

2) Take a fat bladed screwdriver and slide under the black hose and hose clamp and start the pump.

3) This should immediately bleed any air remaining in the pump chamber and you're good to go.