How old is the carbon and what micron rating is it? If its 6 months old or more it probably needs replaced. If its larger than 1 micron I would definitely replace it since it is probably exhausted. A low range chlorine test kit will confirm this. Remember all water goes through the carbon, even the waste so if you have made 500 gallons of RO/DI you have had 2500 or more gallons go through the carbon. Many carbons only last 300 (granular activated carbon)to 3000 (5 micron or larger, or as little as 1000 total gallons for a 10 micron)total gallons. Worn out carbon will kill a membrane and the only way you will know is increased DI usage. What are the tap water TDS and RO only TDS readings?