I have Heatilator HB36AI Fireplace and I am trying to figure out why my fan is not blowing much air.

There is a faceplate that runs the full width and height of the space under the fire box. The fans are mounted in front of approx 3" holes on the faceplate. There are 5 holes and only 2 fans. My question is: Should the extra 3 holes be blocked? I think this will force the air to channel back, up, and over the firebox then out the vents at the top of the fireplace. If anyone can help, I would appreciate it...