A very common problem is a clogged injector. Assuming the timer is running normally and your water softener is using salt at a normal rate, the first thing to check is the injector screen. The owner's manual (see link above) has simple instructions explaining how to disassemble and clean the injector.
If your injector is clean and your water is still hard, then the problem could be the resin beads. Resin beads typically last 10-20 years, but they can get coated with rust and gunk and stop working earlier. Before replacing the resin, consider running some resin cleaner (such as Iron-Out) through your water softener. Follow the manufacturer's directions and mix with the recommended amount of water before pouring it into the brine well. It that does not help, replace the resin (you can DIY for about $100).
The control valve piston can also wear out. If your control valve leaks or the unit gets stuck mid-way through the regeneration cycle, you should replace the seals and piston. Rebuild kits for the control valve used in this unit are still readily available (you can DIY for $50 to $75 worth of parts).
