I would try to restart after inserting the receiver into a usb port if using it. Then with good batteries properly inserted, sync.
also see this document
https://support.microsoft.com/en-us/kb/811798
If your Surface doesn't recognize your Bluetooth keyboard, mouse, printer, headset, or other device, or if the device doesn't work consistently, here are some ways to fix the problem.
Troubleshoot Bluetooth devices