Generally you should expect to remove almost all hardness--but even using SSt-60 resin there may be very small amounts of hardness leakage--perhaps 1-2 ppm or 0.1 gpg.
What are you using to measure the hardness?
What is your maximum flow rate through the softener? Hardness leakange increases with flow rate.
How do you have the control set up? How often do you regenerate and with what amount of salt? What is your typical daily use of water?