Hi Ian
 it sounds like the resin needs changing. 
your tap water is quite  a high TDS. So it will need changing more often 
where I live my tap water is around 65 TDS, and I go through a 25KG bag  Around every 10 weeks.
oh sorry , I misread your post. ididnt see the part where you said you had changed the resin.
sometimes it needs to run a little and it gradually comes down