Record the conditions that show the problem
Write down the heating mode, temperature setting, fan setting, measured room temperature, outside temperature, and time. Note which other heads are operating, any error display, and any pauses or visible outdoor frost. A separate room thermometer away from discharge air gives a useful comparison. Do not open the equipment or measure electrical circuits yourself.
Ask which sensor is controlling the room and what temperature the system reports. Have the technician compare the requested heat with actual operation. That is how we would separate a sensing or control issue from a system that is genuinely at its available output.
Mitsubishi's GS operating instructions describe temporary heating pauses for defrost. That is an example of a normal sequence on those named models, not an explanation for hours of inadequate room heat on another system. Mitsubishi MSZ-GS06/09/12/15NA operating instructions, DG79A04SH01.
Ask for a measured explanation
We would want the qualified technician to check the exact model's control sequence, permitted settings, airflow, sensors, and refrigeration performance. Ask them to identify any current limit, protective condition, or fault and explain its evidence. Defrost should be evaluated through the actual sequence and recovery, rather than used as a blanket explanation for every cold room.
If size is still in question, have the provider compare the room's heat needs and the installed combination's performance at the relevant outdoor temperature. Ask what output is available with the other heads in use. A nominal rating alone is not the comparison you need.
Our next choice depends on the finding. Correct a supported control or operating problem first. Consider a design change when measured operation and the room calculation show that the existing arrangement cannot meet its intended job.
Your next step: Send the log and model information, describe the lost comfort, and request a written performance diagnosis with a practical waiting plan.