I encountered this issue early on with my Series 7. I've been a huge fan of my Belkin 3-in-1 MagSafe charging stand, and the watch puck on that stand just would not play nicely with my S7 Watch. It would charge slowly, and "mess up" my S7 Watch's charging abilities. By that I mean that placing the S7 Watch on the official fast-charge puck that came with it, would charge it over hours and hours. There was a serious bug there. The only way to fix it would be to restart the S7 Watch.
Apparently they tried to fix it in an earlier patch, as this article talks about, but it didn't work for me. My solution was to basically just forget about ever charging my S7 Watch on a non-fast-charge puck. I only use the fast-charge puck and it's been great, although very annoying to have it sit next to my Belkin 3-in-1 MagSafe's puck, and see it go unused.