Maybe sweat, etc from your hands getting onto the pins on the cable and the watch. I sometimes need to wiggle my cable connection at the watch to get charging to occur. While it clicks when connected, I don't think the connection design is as tight as other USB style connections typically are.
You can buy a better charging dock from Amazon, it also comes with little rubber caps that you can cover the charging port up. I had same problem but now it’s solved.
