The OTA update will fail if you modified your stock ROM. I'm not sure what it looks for specifically but if you froze, uninstalled, or modified any of the parts that it verifies, the update will fail. Your best bet is to do what I did. Go to the stock ROM. 1) Download the JB OTA and save it to your external sd card. 2) Uninstall Safestrap Recovery (within the app itself) and then uninstall Safestrap. 3) Download Voodoo OTA Rootkeeper, temp unroot (assuming you were rooted; if not, root, then temp unroot). 4) Fastboot and use SBF using RSDLite and the ICS217 SBF file (or GB if you're still on that). 5) Fastboot and install the JB OTA zip file from your sd card. You should now be on JB OTA! 6) Download Voodoo OTA Rootkeeper again, re-root. 7) Install Superuser/SuperSU (if it's not installed), BusyBox, and Safestrap if desired. 8) Have fun with new ROMs. Keep in mind that only ROMs with the JB kernel or kexec kernel will work. You cannot revert to any ICS kernel ROM.