not able to merge dimensions objects on webi report based on SAP query

Hello

We are using SAP BI 7.0 , BO XI 3.1 sp1

I have two queries in webi report. These two queries are based on the same universe. I am not able to merge dimensions from both queries as merge dimensions functionality is greyed. Two dimensions are of the same type (char) and are dimensions (not the details).

Does anyone know the reason?

Thanks