We were able to resolve this issue by reviewing the console.exe.log from the core which indicated that imagex.exe was unable to access the imgtmp directory found in users\(current user)\app data\local\temp. We renamed th imgtmp directory with a utility called LockHunter because the directory was locked and wouldn't allow us to delete it. Once this directory was renamed the boot.wim mounted properly and we were able to see and inject drivers again.
I had the same issue and renaming the folder worked for me only for the first time I used the WinPE driver tool.
The second time the problem occurred again. I disabled UAC and now it works like a charm.
Nice! Worked like a charm for me.