It sounds like you are doing everything right,, why not fill the firebox all the way up instead of 3/4 full?? Coal likes a deep firebed. The deeper the better..
It also sounds like from your description that you are burning Bituminous coal. Where are you located, and where do you get your coal from.. Hard coal, or Anthracite coal is washed and graded by size, it is washed to remove dust and the sand-sized 'fines'.. Bituminous can be bought as 'run of the mine' which is sand-sized fines all the way to large grapefruit sized pieces
If you add more coal to the fire, you will probably have more fire left in the morning to revilatize your new fire..
If you are burning Bituminous coal,, I'll move this to the Bituminous coal forum..