Remove the valve and take a sample of the resin. Use a 1/2" piece of pvc pipe, push it 5" down into the bed, put your thumb over the pipe and remove a 5" core sample. The top of the resin is where you will find the majority of the bad resin if this is the problem. Take some of the resin and try to "squish" it between your thumb and finger. If it is bad, it will have a mushy feel. It should feel hard, almost like round tiny rocks if it is good. If it is mushy, it is bad. This is common in chlorinated water. Replace the resin with a 10% crosslink resin, or replace with standard resin and install a carbon tank in front of the softener. I would recommend adding a carbon tank. Chlorine is important in a municipal supply to keep the distribution system clean, but once you recieve the water, chlorine removal shold be done prior to drinking, or breathing in thee oxidizing agents.
Do the core sample and let us know what you find. I give it a 95% chance that it is bad resin.