It might be how the text is attached to nets. Verify these settings:
* Top level nets are texted properly, with Virtuoso streaming the text objects correctly to GDS.
* The top level text layer is declared as a Text Layer in the SVRF rules.
* The SVRF file isn't overriding Text Depth. It defaults to PRIMARY, and typically that works fine.
For a detailed discussion of how Calibre applies text to nets, see the Calibre Verification User's Manual (calbr_ver_user.pdf) and look for the section "Use of Text in Calibre Applications." To just interpret the rules you see in the SVRF file, look for the SVRF manual, svrf_ur.pdf. (The manuals should be in the Calibre software tree under docs.)