Q: VGA adapter connected through Thunderbolt Display not recognized

I'm trying to attach a second, ViewSonic VGA monitor to my Thunderbolt display via an Apple Displayport to VGA adapter.  When I connect it directly to my Macbook Pro on the Displayport/Thunderbolt port, it works fine. When I disconnect it and reconnect the Thunderbolt to my Mac and the adapter to the back of my Thunderbolt it does not. System information shows nothing attached to the TB port on the back of the monitor.

Just for giggles, I took a spare TB cable and connected the back to the Mac and it worked fine.

In short.

MPB-VGA Adapter-Viewsonic: OK.

MBP-TB Display-VGA Adapter-Viewsonic: No joy.

MBP-TB Cable-TB Display: OK.

I've reset the PRAM and rebooted. No joy.

  Model Name:    MacBook Pro (OSX 10.10.5)

  Model Identifier:    MacBookPro9,1

  Processor Name:    Intel Core i7

  Processor Speed:    2.7 GHz

  Number of Processors:    1

  Total Number of Cores:    4

  L2 Cache (per Core):    256 KB

  L3 Cache:    8 MB

  Memory:    16 GB

  Boot ROM Version:    MBP91.00D3.B0C

  SMC Version (system):    2.1f175

Roger