Auto rain detect on the Seal U

UK model. Set the right hand stalk up 1 click from bottom, and rotated turny thing 1 click. Expected slow auto wipers that would cope with very light intermittent drizzle. It sort of works but seems a bit random, sometimes does nothing when there's loads of rain to remove, and other times goes nineteen to the dozen without aparrent provocation. Setting the turny ring thing up to 2,3 or 4 again 'sort of seems' to help but it all feels a bit wooly.. anybody else feeling mild frustration with this. Must admit though, once I've got going I found it's easy to ignore the 'problem' when I'm not concentrating exclusively on it at a standstill. Never had an actual real problem with visibility while driving.