I suspect your water is municipally sourced and so will contain chlorine. Constant chlorine exposure will damage resin, but the rate of damage will vary depending on the quantity of chlorine and the type of resin. While resin with 10% cross-linking will better tolerate chlorine compared to standard 8% cross-linked resin, many softeners sold online are often equipped with resin with less than 8% cross-linking.
Damaged resin is the usual reason for the symptom you are experiencing. The correct remedy is to replace the resin.