Your plug-in’s CRhinoPlugIn::ReadDocument virtual override will only be called if the document Rhino is reading contains document user data that this plug-in wrote.
Or am I confused about the problem you are having?
I direct open a 3dm file when first run.
The data of Rhino default type loaded successful.
But override ReadDocument() was no response.
The additional data using WriteDocument() to add in 3dm file loaded failed.
If you put breakpoints in your plug-in’s WriteDocument and ReadDocumentoverrides, are they called? Does your plug-in’s CallWriteDocument override return TRUE?