How are you rendering the maps, what data source are you using? Are you loading the OSM data into a PostGIS database, then reading from there? This is the usual way of using Mapnik.

Or are you using Mapnik to read directly from the OSM XML file? I think this has some limitations, so doesn’t work properly with the standard style files. Some details here: http://wiki.openstreetmap.org/wiki/Mapnik:_Rendering_OSM_XML_data_directly

Otherwise, try asking on the ‘dev’ mailing list. There’s probably someone there that knows how it works.