I had the same issue.. it's not BV6000 fault... the SmartBand may not be visible if connected to app.
Unlink from app, detect save it as Smartlock BT device..
Link in App again. Some operation may need reboot of phone / Smartband.. not all smartband FW has faulttolerant code..
My smartband was Lynwo M2S , HR/BP/O2 works great, sleep mon does not.. app is pretty good.
My BV6000 is the best phone I have had.. love it.
Must give BV sw guys an applaud... it had some issues Q3-Q4 2016 but they fixed it.. with updates almost every month.
With experience from Space HW/SW.. I hope you can keep up the good quality in FW.
Don't let Marketing force to many models.. It's better to sell working phones, then faulty ones with top hw and crappy sw.