I have the same issue with my SER8 8745HS. The behaviour is as follows:
When I plug in the power supply and boot the computer right up, everything works fine. However, if I wait a few minutes before pressing the power button, the Bluetooth device (and WiFi) is not working.
As @Jan mentioned, a USB mouse or keyboard is then required to manually disable and re-enable the WiFi/BT card in the device manager to make it work. Another alternative is to simply reboot the machine.
For now, I have the power button set up in such a way that it shuts down the computer when I press it. This way, I can at least reboot the PC without attaching a USB mouse.
A bit annoying, would really like to know why this happens…