Mac Pro - Need Help With Ram

Hello, I am currently using an 8-core mac pro 2008 model

I have my 2x1GB ram that came with the mac and 2x2GB ram that I bought. Now I have bought another 2x2GB and its not showing up on my computer. I currently have my ram in like this:

Slot: DIMM Riser B/DIMM 1 Size: 1GB
Slot: DIMM Riser B/DIMM 2 Size: Empty
Slot: DIMM Riser A/DIMM 1 Size: 1GB
Slot: DIMM Riser A/DIMM 2 Size: Empty
Slot: DIMM Riser B/DIMM 3 Size: 2GB
Slot: DIMM Riser B/DIMM 4 Size: Empty
Slot: DIMM Riser B/DIMM 3 Size: 2GB
Slot: DIMM Riser B/DIMM 4 Size: Empty

Everything works fine set up like that but I tried putting the new 2GB sticks in DIMM 2 on both risers and it is showing up as empty, does anyone know if im doing something wrong to make these show up. Would be much appreciated if someone could help me out, thanks.

The MacPro has specific requirements as to what riser slots memory should go in based on how much you have. Did you consult the user manual as to what slots should be filled on what riser and in what order?

http://manuals.info.apple.com/en/MacPro_Early2008_MemoryDIMM_DIY.pdf

If you did this, try running the hardware test that came with your install disks and see if the sticks show up as bad. It doesn't happen often, but from time to time you could get defective sticks.

I got it all working, that manual was a great help thanks alot for posting the link!