Once the pipes are hot, take it for a drive, fast enough for air to pass through the intake. If your heater core is indeed not clogged or if you don't have a air bubble in the heater pipes you should feel warm air come out of the vents. If that stream of air is not warm while driving you have other issues.
It could be a duff switch, did you have at any time a problem running the fan, did it refuse to run then when you fiddle with the switch it worked, if not it could be a duff fan motor, if your pipes are warm that feed the heater then its circulating fine, firstly check the wiring going to the heater not easy I know, check the plug it could be grubby.