When calling the subUserHistory endpoint, make sure that you’re using your main account’s API/Secret Key, not the Subaccount’s keys. Please let me know if you’re still having issues seeing the transfer history into/out of your Subaccount(s) once you’ve ensured you’re using the main account to make the request.
Ok I see. Then you would just treat the subaccount as if it was a regular account and use the standard transfer history endpoint (GET /sapi/v1/asset/transfer): Binance API Documentation