I got the Fitbit Sense a little bit back and have been having GPS problem since I got it. Turns out it may have been due to starting my first workout in Nashville, second in Oregon, and subsequent over 3 other states.
So there’re two solutions for the GPS problems I’ve had – the first is go outside, grab yourself a lemonade, choose exercise, walk or run or anything that uses the GPS, and just drink that lemonade. A few minutes in (up to 12,) you’ll get your GPS lock and any subsequent locks will probably be in the 8-12 second range. Something about time to first lock.
The other solution I ran across is to make sure your phone isn’t set in power saving mode during the first GPS lock. Not sure if the watch pulls additional data off the internet or what but as long as that was in power saving mode with the screen off I never got the first lock.
Neither of these may be your problem, but both were mine.