At first, I used three sticks of 4GB RAM for 12GB total, installed in slots 1, 3, 5.
Once I added three more identical sticks, slots 4 and 5 were not fully recognized in the BIOS: the 24/16 message is seen under Windows (7 Ultimate 64 bit).
The memory sticks are Patriot Viper II Sector 7 PC3-10666 1333MHz. 24GB kit; all six sticks identical.
I believe the CPU and heatsink were properly installed.
A change of voltage from default to 1.6 made no difference, other than getting the "recovering DRAM size" message and the machine re-booting several times.
What else is there to do?
I have a pic of the BIOS page showing the status of the last two slots if needed.
Thanks for any feedback, Rick