I was actually able to replicate this issue in a test environment. Spun up 3 brand new 2012R2 servers, loaded VBR on one, windows agent on the other two. Setup repositories, WAN Accelerators and backups. Just using defaults for each. Next, next, next.
Selected some local folders on the Windows Agent servers, created the backup, ran the backup copy job and 0.0 B pulled from global cache. Populated cache and still the same. 0.0 B from global cache. edit *c:\windows was one of the folders and when populating the cache the population job showed the OS as found and populated successfully*
The I added the system state to the agent backup jobs. Next copy jobs pulled 9GB from global cache and worked as expected.
It seems the system state needs to be included in windows agent jobs for global cache to work when running a backup job for them. Odd.