It's been happening for sometime that I can't get Windows 10 to hibernate, and I think I actually figured it out.
I have a dual-monitor setup and if I have windows open in the secondary monitor Windows 10 will refuse to enter in hibernation mode. To my surprise simply moving all the open windows to the primary monitor, and then clicking the hibernation button fixes the problem.
Of course, if you don't have a dual-monitor setup, it's likely that a running application is preventing Windows 10 to hibernate. In this case simply make sure to close your applications one by one until you pinpoint the one causing the problem.