Can't get water below 3.5 GPG hardness with water softener

Hi all,

We recently installed a water softener that we were given by my sister-in-law. It is a Fleck 5600SXT that is about six years old. We had a plumber install and setup the system. Since the install our water is softer but never gets lower than about 3.5 GPG. My incoming water is about 16 GPG. I have checked to make sure there is no salt bridge. I have manually regened several times. I've also even replaced the resin with ResinTech CG10. I am running out of ideas to try other than completely replacing the system. Any help would be greatly appreciated.