One "trick" I have found (archaic I know, is to empty all fluid out of the tank, hold the tank hose in the air (prop it up) and fill with oil. The hose must be above the pump. (marvel mystery oil works well). Then tap the pump with the back of a screwdriver. Have a friend try to engage the pump while tapping.
Often pumps are "stuck" from idle time and detergent oils will free them. I do warn if the pump engages that you WILL get oil on your windshield so be sure to have a degreaser and window cleaner available.