Could you please provide more detail. Note the "processable" flag does not mean the content you specify will be parsed, but processed. This means if you specify a String, that will be considered a Text (not parsed for tags inside it), but any inlining inside it will be processed.
If you need to actually create an IModel (i.e. actually parse some template code), you should be applying one of the parsing methods at the Model Factory (context.getModelFactory()), and then specifying the resulting IModel as the content passed to the other version of structureHandler.replaceWith (the one receiving an IModel argument).