I had the same issue last week. flushed both ways with water while having the hose shoot into a clear jug to see what I caught. Water didn't do much. I tried water then blowing the hose the same direction with an air nozzle. Water first then air-Short bursts of air- don't want to make it a balloon. I could not believe what came out. That heater core makes for a good filter. I will note that I had both hoses hot going to the core before this so I knew I some flow. If you have done a plenum gasket, or water pump like I have you will probably find chunks of gasket, strips of RTV silicone, maybe sand. bunches of stuff it has collected.

Mine heats very well now.

 

Haa, yeah, did that the other day after getting annoyed from the buzzing sound that wouldn't go away. Emptied out all the helicopters.

 

Well, thanks for the tips. But here is what finally happened. Let the boy borrow the truck for work on a nice 50+ degree day and of course forgot to tell him to remove the cardboard! Overheated the truck, thought he checked the fluid level correctly in the dark without a light!!! Drove it home. I checked it in the morning and only 1.5 gallons low! After topping it off heard a crackling noise from the heater core. Broke free a ton of rust and crude. Of course jammed up the thermostat. Reflushed entire system and back flushed the heater core. New heavy duty thermo from NAPA and Walla heat ever since.
Who would have thought your kids screw-up would actually fix something, maybe is a mechanic and not a bookworm!
Thanks to everyones input and for those of you that are still having the same problem....Let your Kids at it! Persistence will pay off!

 

Moral to the story then is, if you have no/low heater output, it's probably the heater core and you may get lucky back flushing. Poor radiator cap or air in the system will cause engine operating temperature to be higher than normal and not affect heater out put.