I used to own/operate a CS company. I'm likely the most qualified person on this board to answer your question. And the answer is no, altitude does not matter.

If you say, "nothing is happening" I assume you want the water to change collars, or some bubbles or something to appear — right?

To do it right, you'll need a PPM meter. Check the PPM of your distilled water (should be 3PPM - 8PPM) prior to introducing the silver-electrodes and 12V DC current. Continue to run current until the PPM is up to 21-24PPM. Depending on how much water-volume you have, size of electrodes, surface area of electrodes, etc., it should take 2-6 hours.

If you have no PPM meter, you can determine that it's "done" when the water goes from clear, the slightly hazy-gray, or just taste it. It should taste like a silver-spoon in your mouth.