You must abandon the official update path and look to the developer community.
If your device is currently running Android 4.2 (Jelly Bean) and you want the official last update, you might be able to manually push it if the OTA isn't working.
If your tablet won't update via Wi-Fi, download the Lenovo Rescue and Smart Assistant tool on a Windows PC.
Breathing New Life into Your Lenovo Yoga 8 (B8000-F) The Lenovo B8000-F, part of the original Yoga Tablet 8
Connect your B8000-F via USB. The software can often "rescue" the device by flashing the latest official firmware available in Lenovo's database.
If the technical steps above look like a foreign language, you have three alternatives: