The TT RDS system has been notoriously poor in the UK, works ok in europe.
First make sure the unit came with the latest TMC 4V00.013 (or 9V00.017), if it didn't then it's old stock and I would take it back, the newer versions are better than previous versions.
You may need to manually tune to classic FM wherever you are (you only have to do this at the beginning of the journey as it should change automatically after that).