This might be stating the obvious but just in case, you almost always have to remove your old pairing before it detects the new version. I try various rom versions often and mine has never paired if there is an existing pairing. You must delete the old pairing and let it detect and set it up all over again.
Once I did get two devices to show up but sync center didn't respond after that.
Also it should be stated that many times when deleting a pairing it doesn't actually go away and I must reboot in order for it to actually remove. If this happens and I don't reboot it stops doing its job but still responds to mouse clicks. I have tried restarting the services to no avail and seems only a reboot fixes.
My experience has been with my phone with about 6 different ROM versions and with my team's devices as I upgraded each one (~5 devices) and I found each one required me to remove the old pairing first and re-setup a new one each time.
Hope this helps,