Hi
I am not sure whether you are using IS-Mill product where you have option to propose Auto GR for co-product and manully qty to be entered.But in SAP ECC 6.0 EHP5 , Auto GR of Co-product is not possible .But you have to go for custom txn to call CO11n using BAPI_Goodsmvt_Create which can help to trigger 101 based on FG, Co-product .Checki with your abper
Please check OSS note 38108 and 943522 which may be helpful.