This seems to have just begun recently (within the past month) where AME CC 2014 is only using 1 core (out of 12) to render video. No, this is not the CPU Parking issue - I have made the regedit tweaks and also changed my Power option to Performance, all with no difference in AME. Premiere Pro CC 2014 renders the same video just fine using all 12 cores, so it seems this problem lies not with Windows software, nor Premiere Pro, but just AME CC 2014. Was there a recent AME update that might have included this bug?
Adobe, please look into this and resolve it ASAP. Not being able to queue up video renders is killing my productivity.