At the risk of sounding like too much of a nerd, you should always use distilled water for you QD mixes. All tap or well water has minerals in it, typically referred to as it's "TDS", or "total disolved solids". Depending on where you are in North America, this can be as low as 30 TDS (soft water), up to 800 TDS (hard water). Most minerals found in tap or well water consist of calcium and flouride. Both are incredibly unforgiving to surfaces, especially as they dry. Think about your shower walls and door, or the sprinkler water drying on your paint. Mixing this with a QD is doing the same thing, albeit at a lower rate. No matter what you do, you will still be leaving some of these minerals on your paint, hence the lack of "slickness".
Distilled water on the other hand has a TDS of 0 (or very near to it), therefore you only leave the polymers on the paint if using it in a QD. The slickness you feel are the polymers, and the LACK of minerals from using distilled water.
Twenty years in the bottled water industry, and I now know more about water than I ever need to.