I also have a room that does not cool/heat as well as the other rooms in our house(about a 4 degree F difference). The furnace and AC are forced air. We actually leave the door open to allow adequate air return. Would adding a higher output fan be a possible solution?

Chances are that it won't help. Chances are that you have a central return. Closing the door restricts the return from pulling air out of the room. You can't put more air into the room unless you take some out. Leaving door open or otherwise allowing more air to be pulled out is only answer (short of doing something really radical). You could try cutting off the bottom of the door, thus allowing more air to pass.

Wow. Thanks for the quick response!

I did want to clarify that I'm seeing a temp difference of 4 degrees with the door open. I don't mind leaving the door open to allow return, but would like to push more air to the room from the heater/ac fan. Holding my hand over the vent, the air-flow seems really weak compared to other rooms upstairs. I have checked for obstructions (mirror and flashlite) and can't see anything. I would like to try something short of ripping out sheetrock and re-venting? Thanks in advance for any help/ideas!