z400 - 6 slot memory configut=ration

I am using a HP Z400 Workstation Xeon Quad-Core W3565 3.2GHz, 12GB (3x4GB), 1TB, DVDRW, Windows 7 Pro.

This the the model with 6 memory slots, so there was a possiblity for 24GB of RAM.  When I populate the remaining slots 4, 5 & 6 with 4GB DIMM, I get errors on boot up for the newly populated slots.  I am unable to configure for 24GB.

I did discovered that the DDR3's that I am using are non-ECC.  The system has been running fine with the (3x4GB) setup.  Is the non-ECC preventing the 24GB configuration from working?

Thanks for any help you can offer.