I assumed you have cleaned out the grates, holes and UNDER the grates good. You will get fines built up under the grates.
Also, air blow off the fins on the fan, keep them clean too. Use canned air, toothbrush, etc...to clean the fins.
YOu could open the intake on the fan a bit, but make sure you monitor the draft with a manometer (Draft gauge) to make sure you are not pressurizing the heat chamber and cause CO to leak out. YOu may have to adjust your baro damper or power vent to accomodate the higher air flow.
My hyfire doesn't even have covers on the intake of the combustion blowers and burns pretty good.