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.