There could be a few reasons for this, your door may need new seals. You may need extra flue to make your fireplace draw better OR your wood might be too wet.

A large fire box is designed to be filled up to get the best effect of heating. Try filling your firebox full of kindling getting a big bed of coals and then fill your firebox completely full of wood. From there you should only need to maintain the fire while having your air control on low allowing a longer lasting burn time.

You can clean your wood heater glass using any nonabrasive cleaning product OR you can use the ash in your firebox on a wet piece of newspaper, ensure the ash is really fine so you do not scratch your glass.

Once every year or a year after using a qualified chimney sweep.

Any hardwood, or any mix of softwood that is not treated and under 19% moisture.

We recommend at least an inch of ash be left in the firebox.