See-through fireplace overheating on one side

I have a see-through fireplace from Heatilator - the front side in the living room, the other in the bedroom. After only 15-20 minutes the back side gets extremely hot while the front side is not all that hot. This extra heat is so bad that it causes the black (hard) plastic directly above the fireplace opening (still part of the Heatilator) to emit a smell like plastic burning. It's so disagreeable that we have to turn the fireplace off. It's also probably a good idea for safety.
It seems that the heat from the gas (propane) log fireplace is primarily focused on the back upper wall of the fireplace - thus causing the overheating. Does anyone have an idea what the problem is, and the fix?
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