@SquidgeyMama - the only thing that worked for me two times now was to not use it for multiple days in a row. NOT a great fix. I originally tried all the "fixes" and not one of them worked. Deleting and reinstalling the app, worthless. Deleting the actual device from the app was NOT good as it caused more problems than it fixed, so that one I would say not do. Restarting phone didn't work, didn't hurt, didn't help. Deleted my WIFI, not sure where that one came from or why it was supposed to work, but that didn't do a thing either. I can't remember but there were a couple other things, all which did not work. I was in the middle of a challenge when it happened so it was really irritating to the point that I decided to just stop using the Charge4. I still had my old "one" so I connected it back to the app and it worked fine. I forgot about the charge for 2 days, which is all the amount of time my old ONE can actually hold at this point, LOL, but hey it's the original (and I knew it wasn't holding the charge, which is why I bought the newer device in the first place, too bad Fitbit didn't keep that, it was and is still the best one they ever made, LOL) Anyway, after not using the Charge4 for those two days, it then started *magically* working again for about a month, before it did it again, whereby I did the same fix - don't use it for a day. It seems that was the only fix I could find that actually worked. Plug it in to the port and leave it for a day of rest and eventually it reconnects, until it breaks again. Not ideal, but Google can track your steps through your phone on the day you give the Fitbit device a day-off. Good luck with your marathon.
