As Lisa has indicated, more information is always more useful in helping folks out with their problems.
If you would add your location and type of stove/furnace you are using to your profile, we would know what climate and outside temps you might be experiencing. Also in coal discussions we would know what coal breakers and dealers are in your area.
I agree with everything Lisa said, and would emphasize filling the firebox to the top of the firebricks. Having the deep bed is critical for a long burn.