BT wont' reconnect automatically

Hi, I have been using a Gear S2 for some time without problems, but I have just purchased a new mobile, a UMIDIGI Z1 Pro and although the Gear connects via BT, when I go out of range and return, it won't automatically reconnect. I have to push the power button on the mobile and the two connect! What settings on the mobile could be wrong and preventing the BT automatically connecting? Thank you