If you go to ASUS' site for your motherboard, and look at the specs under memory, they have a note:
*AMD AM3 100 and 200 series CPU support up to DDR3 1066MHz.
http://www.asus.com/Motherboards/AMD_AM3/M4N68T/#specifications
Thanks, for the advice - tried what you suggested, still no go. The old memory stick still works.
Wondering if it could be my processor - it's only an Athlon II X2 245.
It could be the CPU. By the way, please try and use the edit button. It's against the rules to post subsequent times in a row. It cuts down on extra thread bumps.
+1: I would imagine that it would normally underclock the memory so may a BIOS flash could fix it, but if other memory modules are working maybe it's having trouble trying to default to JEDEC spec for 1600Mhz.
This might sound a bit odd, but try putting your old memory in A1 and B1, and putting your new memory in A2 and B2. The CPU might detect the old memory first and down-clock everything to the slower speed. I'm not incredibly confident that this will work, but it is worth a try.
Check to see if there is a BIOS update first though before you do anything else.