I have this problem constantly with my iPhone 5c. The bluetooth is good - so good that it connects to my car and starts playing music when I'm 100 feet away (even if I don't want it to).
With the Band I have this issue and have seen it across a replacement now as well. Basically I have to poke and prod my Band and Phone about 3 or 4 times including a restart before it will finally start Syncing.
Needless to say this is frustrating if you are running and want to lock GPS - have a sweaty finger and cannot swipe the band screen for the Bluetooth settings to give it a proper kick in the pants to Confirm "Keep Trying?"