Hi Anil,
I assume your copying control setting are correctly in place. Provided the custom sales order type are copied from the standard sales doc type.
What is item category(I Cat) and schedule line category(Sch Line) you are using in your Return Order? If you are using custom I Cat or Sch Line, please provide the screenshot for the same.
I hope you are aware I Cat controls the behaviour of material/item in the sales order. Whether item is relevant for return or not. Whether Item will have a delivery related billing or order related billing. Therefore, in the Business data section of your I Cat check Billing Relevance and Returns indicator.
Thanks, JP