The error codes' meanings are:
14 - Abnormal accelerator Hall, please check accelerator twist grip, instrument panel and related wiring.
15 - Abnormal brake Hall, please check brake twist grip, instrument panel and related wiring.
39 - Built-in battery temperature sensor abnormality, please replace the battery.
If they do not help pin point the issues then:
1. What firmware versions are on devices? Are they different between working and not working ones?
2. Are configurations same between working and not working devices? (could you send me the configuration you are using?)
3. Do devices that do not work show same errors on scooters which work with other devices on?