You say you are running sub bituminous which is coal that hasn't quite formed to bituminous coal yet. It has a lower carbon rating and lower BTU output/lbs as you can see in this chart. http://geology.com/rocks/coal.shtml
A mass of recently accumulated to partially carbonized plant debris. Peat is an organic sediment. Burial, compaction and coalification will transform it into coal, a rock. It has a carbon content of less than 60% on a dry ash-free basis.
Lignite is the lowest rank of coal. It is a peat that has been transformed into a rock and that rock is a brown-black coal. Lignite sometimes contains recognizable plant structures. By definition it has a heating value of less than 8300 British Thermal Units per pound on a mineral matter free basis. It has a carbon content of between 60 and 70% on a dry ash-free basis. In Europe, Australia and the UK some low-level lignites are called "brown coal".
Sub bituminous coal is a lignite that has been subjected to an increased level of organic metamorphism. This metamorphism has driven off some of the oxygen and hydrogen in the coal. That loss produces coal with a higher carbon content (71 to 77% on a dry ash-free basis). Sub bituminous coal has a heating value between 8300 and 13000 British Thermal Units per pound on a mineral matter free basis. On the basis of heating value it is subdivided into sub bituminous A, sub bituminous B and sub bituminous C ranks.
Bituminous is the most abundant rank of coal. It accounts for about 50% of the coal produced in the United States. Bituminous coal is formed when a sub bituminous coal is subjected to increased levels of organic metamorphism. It has a carbon content of between 77 and 87% on a dry ash-free basis and a heating value that is much higher than lignite or sub bituminous coal. On the basis of volatile content, bituminous coals are subdivided into low volatile bituminous, medium volatile bituminous and high volatile bituminous. Bituminous coal is often referred to as "soft coal," however this designation is a layman's term and has little to do with the hardness of the rock.
Anthracite is the highest rank of coal. It has a carbon content of over 87% on a dry ash-free basis. Anthracite coal generally has the highest heating value per ton on a mineral matter free basis. It is often subdivided into semi-anthracite, anthracite and meta-anthracite on the basis of carbon content. Anthracite is often referred to as "hard coal"; however this is a layman's term and has little to do with the hardness of the rock.
My guess is, the sub-bituminous burns up quicker and doesn't hold the heat as well as bituminous and anthracite. When I burned bituminous coal it help high temps the entire time. If closing down the manual flue damper isn't working then it must be the low BTU coal you are burning. Try bituminous or anthracite and see if you notice a difference.