Neither Windows 10's built in Bluetooth nor WiinUPro can seem to locate my newly bought first party Wii U Pro Controller. I have pressed and held down the POWER and SYNC buttons many times over now, and still nothing.
I have this same issue (also Win10) with my third-party wiimotes, 2 different manufacturers. I ordered a third-party WiiU controller off of Amazon but worried it won't work either.
Seeing as you're having the problem with first-party stuff, I'm feeling like the problem is with Windows 10/the bluetooth stack, as the controllers should use the same syncing protocol to connect to the same first-party console no matter who makes them.
Do you have a built-in or USB bluetooth adapter? What's the manufacturer of the dongle? (Mine appears to be Qualcomm.)