Motherboard can't read memory module

We have a desktop HP dx7300 Microtower that wont read any memory module that I put on the third slot which is dubbed XMM3 on the motherboard schematics. These memory modules that I mention all work on the 1st, 2nd and 4th slots but not the third.

I have 4 memory modules: two DDR2 667s and two DDR2 800s. I tried pairing them, like 800 on 1st and 3rd and the 667 on the 2nd and 4th but still no go. Hoping for your reply thanks.