My question, in order to make a suggestion is, what is the size of the coal you are using?
Even before quality of coal is considered, you need to have the proper size of coal.
Smaller size coal will burn slower, not yielding alot of heat but lasting a longer duration than a larger size.
Where as, larger sized coal will burn faster, with greater heat output for somewhat shorter duration.
Sounds like you may be using a smaller size coal compared to previous. Or maybe you're burning "Run of Mine" (ROM) and it would tend to burn as smaller sized coal even though it has some larger pieces in the mix.
Coal burns faster and hotter as the combustion air is more freely able to pass up through the coal bed 100% from underneath.
In short...faster, hotter...equals easy ability for the air to go pass through glowing coal bed.
Then you only need to have a means to control or slow down the rate of burn by a couple ways.
Adjusting air amount allowed in from bottom or adjusting the amount of exhaust gases before they exit...control both is even better.
Hope This Helps