Why is the temperature in a commercial oven uneven?

Please verify the following points and require itemized written supplier responses:（commercial oven oven-uniformity） Uneven temperature may result from overloading, blocked airflow, fan failure, aged elements, leaking door seals or inaccurate controls. Test each level with thermometers, leave airflow gaps and replace faulty parts. Define capacity, accuracy, materials and operating environment before requesting quotes on a uniform basis. Request specification sheets, test reports, certificates, maintenance manuals and spare-part lists. During acceptance, verify appearance, dimensions, functions, safety protection and packaging, and keep test records. Do not compare unit price alone; include customization, freight, spare parts, after-sales support and downtime risk. Keep written records of inquiries, confirmations, FAT, acceptance and warranty commitments.