Same thing was happening to me as well... i paired and unparied, factory reset multiple times, uninstalled app, reinstalled app ... this last time it just started working ... who knows ...
the full process was: unregistered band from within app, uninstalled app, deleted bluetooth device from phone, restart phone, factory reset band, paired band to phone from bluetooth menu, when i tapped on the device it would connect but stay connected for only half a second at a time ... then i installed the app, added the band, and it is working (it is staying connected, and keyboard / cortana are both working).