I'm having the exact same problem. I changed the impeller looked at the lines, etc. I have taken my thermostat off and tested it in hot water and it is opening but I'm not sure if it is all the way. Since I already have it all apart I'm going to go ahead and put a new one in there. I'll let you know what I find out.

BTW, I do have a clear strainer and it is pumping water but I'm not sure of how much the flow should be. Someone told me that I should be able to put my hand on the exhaust for a couple of seconds before taking it off.