Upon arrival, unit in question was set in heating mode with no operation. Other hear was operating in cooling mode to try and cool excessively hot other room. Set other unit to off and turned head unit in cold room on. System came on and fan started to blow. Supply air was slightly warmer than return air by approximately 2F. Inspection of outdoor unit showed that vapor line was getting to 130-140F which is normal. Refrigerant coming into head is not 130-140F and is approximately 70F. Suspect that there is an insulation issue in the attic above garage but the garage attic cannot be reached currently due to needing a 10ft ladder, and there being stuff in the area the ladder will need setup. Currently have the head in question in heating mode with the other head off. Multiple issues including the other head being in cooling, very low outdoor temperature causing reduced capacity and more frequent defrost cycles, and being a bonus room above the garage can all affect the heating operation.