Activated carbon is like a molecular sponge, except that it adsorbs contaminants at different affinity rates. In other words, it "likes" certain chemicals more than others. If the "sponge" is full and a more attractive guy comes through, it can toss out some less attractive bloke to make room. That particular contaminant may not be to your liking.
Find out the age of the tank and when it was last regenerated, and hire an expert to get the system serviced.