You have to select the Group and on the Option bar select place detail group then it will attach to the group. When you saved the group out to a new RVT file, did you remember to select the Detail Group as well. Model and Detail Groups are two separate objects in Revit and each must be selected to have them both.
Mel Persin, AIA
AEC Technology Consultant
Technology to Visualize and Realize Solutions
Modeling for the Future/Drafting in the Present/Building on the Past
Mel Persin, aia