Q: Coolant not flowing.

I have a 2006 Nissan Frontier, The oil cooler went bad about two months ago which caused the oil to mix with coolant. We replaced the oiler cooler and flushed the system. About a month later the same problem came back and this time the car actually overheated on me. When I pulled the radiator cap it was filled with oil again, only to find out that the new oil cooler went bad again. The cooling system was properly flushed by a professional radiator Place and just today he fit everything back up including a new radiator and informed us that there was no coolant flowing. What could be the cause of coolant not flowing? Could the oil have damaged the water pump, or could there be a huge blockage in the actual engine? He must have been getting proper flow when flushing it so I'm Assuming it's the water pump.
My car has a manual transmission.