Windows 10: primary monitor taskbar not hiding
I had this problem after upgrading from Win 7 64bit to Win 10. What I found was that Microsoft's OneDrive app was causing the issue. Once I exited OneDrive from the hidden icon area, my taskbar went back to normal. I've since uninstalled OneDrive completely.
More specifically...
The Win 10 taskbar refuses to autohide if there's ANY notification "barking," and that's the way to attack it. Settings / System / Notifications and Actions. First, go to Notifications and turn off "Show app notifications." If that solves the problem, then you KNOW it's one of your "notifying" apps. Then just turn "Show App Notifications" back on, and focus on the app notifications one by one.
Back to OneDrive... in my case it is OFTEN OneDrive. You should be able to click it and choose "Repair" and at least you can make it happy. Or just disable OneDrive's ability to block the taskbar autohide.
In Task Manager, try to re-start the 'Windows Explorer' Windows Process.
Do you have the search hidden on the taskbar? I was having this same issues that persisted through reboots, but re-enabling the search icon/box and then disabling it again allowed both taskbars to auto-hide.