We have encountered this issue on the HP 810 G1 Revolve. During BitLocker activation it verifies the ability to insert the PIN in the pre-boot authorization environment.
For some reason the keyboard wasn't identified. You can verify this is the issue by looking at the MNE logs.
There is a workaround using GPO's.
Posting the logs could help verify that this is the issue or help detect the issue if it's something else.
Thanks for your reply, however, the solution, for me, was the following:
Run gpedit.msc then go to Local Computer Policy/Computer Configuration/Administrative Templates/Windows Components/BitLocker Drive Encryption/Operating System Drives and enable use of BitLocker authentication requiring preboot keyboard input on slates.
I should add that this was being attempted on a Surface Pro 3. After changing the above local computer policy, I was able to control BitLocker on the Surface Pro 3 running Windows 8.1 Enterprise. Hope this helps someone in the future.