I think that your problem is a likely to be a result of significant quantities of rust sludge building up in the heater core and blocking it.
Once this happens, no amount of 'flushing agent' will work because it doesn't get the chance to flow around the heater itself......and to be honest, the stuff in those bottles will NOT dissolve rust sludge - no matter how long you leave it.
The best I can suggest is that you follow the procedure I detailed at the beginning of this thread to shift the muck out of the heater core. Be prepared to do it a couple of times - the amount of muck in ours was quite surprising and managed to re-block the core after a couple of days, necessitating a second 'go' at it.
As long as the heater core isn't perforated (punctured /leaking) at any point this technique stands a good chance of working providing
you can initiate some sort of flow using a hose. DON'T use any sort of pressure washer on the pipes - its likely to burst them.
Avoid any sort of 'radiator sealer' stuff at a later date, in my experience this stuff usually causes more problems than it solves (blocks things up!)(IMHO
Lots of detail and pics at the start of this thread as to what you need to do........
Best of luck