Your water is a likely culprit- if it is acidic to begin with, or quite soft, with lots of organic acids in it like tannins, you will find that your soil goes bad FAST. The real kicker if you happen to have organic acids is that you wouldn't necessarily notice them since many common ones in surface waters buffer in the high-6 range and go unnoticed until you see soil problems.
I'd deal with your issue like this:
-Maintain a frequent re-potting schedule so fresh roots always find fresh soil, being sure to do a good flush with pH adjusted water a couple days before repotting.
-Check the hardness of your water and its pH to try to get a sense for how it might be interacting with the soil.
-Plan to water with a nutrient solution that you KNOW to be of correct pH for your garden, and if your water is soft, run CalMag.