Heater blows cool air

2001 Chevrolet Blazer with 70,000 miles. Heater started blowing lukewarm air.
I disconnected hoses to heater core and flushed it with a garden hose in both directions until water ran clear from both ends.
I also topped off radiator and let engine run with cap off to let any air out of system before replacing cap. Had hot air for about 1 minute after flushing core, but now its cool again. When I feel the hoses with engine running, the hose going in is very hot, but the hose coming out is cool and feels empty. Haven't found any leaks. Could this be a faulty core or is something else the culprit here? The engine seems to to never get near the halfway point (240 degrees) even when I run the engine at 3,000 rpm's in the driveway.