Is yours a gas start that you use with real wood or is it a gas log fireplace (fake wood)? If it's a gas log fireplace:
There are 2 types of gas logs that you can have in your fireplace: vented & vent-free.
The vented logs require your fireplace flume to be open while the fire is on. If yours is like this, then most of the heat will escape thru the flume. The fire will be pretty, but may not produce too much heat for you.
Vent-free logs do not require the flume to be open, so the heat moves out into the room much better. Pretty & heat producing.
Hope that might help explain your problem!