You might check the hose between the inside of your transom and the pump....

If the hose is directed "up" from where it connects to the transom and then back "down" to connect to the pump.....this creates an air pocket in the hose that the pump will not pull thru the pump...thus...no prime...

Try to direct the hose in such a way that this air pocket will not form between the transom and the pump....