If you have fresh coal on the fire, the manual damper open, the loading door closed, the ash door open, you should have the fire getting hotter, If you have a piece of 1/4" or 3/16" steel rod, bend an 'L' on one end and use this to poke up from below through the gaps in the grates. This will open up the air passageways.
You may have waited too long to add fresh fuel and have only glowing embers remaining, and they don't hold enough heat to relight fresh coal.. You may have to start over again with a fresh fire. If the heat output of the fire is too low, you won't get much draft from the chimney, and this makes recovering the fire even more difficult..
Do you have much of a fire or heat in the furnace ?? Is the chimney flue pipe hot? or just luke warm.?
Sorry, I can't think of anything else right now.