Why can't I keep my wood stove running?

I have an enerzone solution 1.8 which I bought last year... I'm fairly new to wood burning, only been doing so for a few months now. The problem is I seem to have trouble getting a good bed of coals to last more than a couple hours. When I put a large piece of wood on the fire it usually won't burn and just smothers out the coals. I can have a great fire going for most of the evening (with small pieces of wood), spread out the coals before bed, put on a couple large pieces of wood, and they usually won't ignite. Not exactly sure if damper position has anything to do with this? Usually if I put on a large piece I will leave the damper open fully and leave the door slightly ajar for a little while. Should I be closing the door and closing the damper instead? Yes, my wood is fully seasoned, my chimney is clean, and is at the proper heighth for air flow. Some advice from an experienced wood burner would be appreciated since I can usually only burn small pieces of wood which won't last more than 4 hours. Thanks for the help.