I've had this issue on my F5 as well. I've found that sometimes I have to disconnect from my phone's bluetooth in order to get a successful calibration. Not sure why but it seems to work...... and I don't always have to do it, but when I do get a few failed cals in a row I disconnect and it works the next time around.
Had the same problem today. The figure 8 thing pictured on the watch didn't work for me no matter how many times I tried. I think the main goal is to get it to move around in a bunch of directions in an unreasonably short amount of time. I took the watch off of my wrist and held one strap in each hand, then twisted the bands in random directions and it calibrated successfully. It also calibrated itself when I tossed it into the air and have it hand on a bed or something. Kinda silly.
