Not everything is as it seems -- Exporting Revit parameters to IFC

Ok, this is a bit counter-intuitive but:
If you want to export parameters from Revit to an IFC file, note that the parameters should not be grouped under the category "IFC parameter", but under a different (eg, .. ID data, text) category.

This is because the IFC parameter group is for inbuilt / standard IFC parameters.

via
Autodesk BIM Blog - Tipp f�r IFC-Parameter Export f�r Revit Produkte