You've probably already tried this but it works for me.
Where the charger goes onto the phone there is a groove that helps the magnets to locate properly.
This gets fairly well packed with sweat, grime and so on which can pack in fairly tight, just wiping doesn't remove this and often stops the contacts being able to fully connect to the lead.
Use a pin or a needle, I find the eye end of a needle works well, and give this groove a good scrape out then once clear use a toothbrush to give it a scrub.
I wouldn't use any liquids, apart from isopropyl alcohol, as the waterproof rating is pretty lousy on these ionics.
I get the power supply message a lot because of my work being pretty messy and a lot of debris gets behind the watch causing a poor connection.
Worth a try as it's a possible free solution that's really only a deep clean.
