If it were mine, I would pull off the l.u. again and do a couple of things. Test out impeller operation in a tub of water, just to be sure (can turn the shaft manually clockwise with a hand drill, and make sure you get a flow out of the water tube grommet in the housing). Then I would find a way to pump water up the tube through the powerhead -- hose with the male end of a quick release can work, maybe with a piece of tubing. See where the water exits. Probe up the exhaust port in the leg (wire clothes hanger or something), and see if there is a blockage. Think wasp or mouse nests.
If it's a salty or a river runner, there may be salt or sand blocking a passage. You might clean it out with the manual flushing, but may have to remove the head cover and clean the passages -- maybe the exhaust cover as well.