The most common reason for leaking is that the tubing did not get pushed in all the way, and the O-ring is not sealing properly. One trick is to measure 3/4 an inch and draw a line on the tubing going into the fitting. That line should disappear into the quick connect fitting.
Another reason that you may be experiencing a leak is that the tubing was not cut properly. If the tubing is cut in a diagonal shape rather than a square cut, the O-ring may not have been able to seal all the way around the circumference of the tubing.
Another potential issue is a lacerated O-ring. If the tubing doesn’t get cut properly, and it has a sharp point, the sharp point could damage the O-ring, making it impossible to seal.
Too much side torque, or side load, could also result in a leaking fitting. If you try to bend the tubing to make a connection to your water treatment system, the tubing itself could elongate the O-ring into an oval shape that will create a lapse in the seal and cause the fitting to leak.
Frequently taking the tubing apart and putting it can cause gradual damage to your tubing. The metal teeth on the inside of the locking collet bite hard into your tubing. Every time that you remove the tubing from the fitting, you could be scraping away small bits of the tubing. Over time, this frequent removal could cause your tubing to be scratched so deeply that it bypasses the O-ring, allowing water to pass. If you are in need of a fitting that you can connect and disconnect over and over again, you might want to consider using a quick-release fitting.