Vostro 3900 only 1 RAM slot working and cannot upgrade memory

Hi everyone, i have a Dell Vostro 3900 i3 desktop and have a 1x4gig ram. Now i bought 1x8gig to use alongside the 4gig and the problem is i cannot use both at the same time because only one (slot 1) shows up in the Bios/Properties, and it wont read the other one (slot 2)...

Both RAMs work well in slot 1 but not in slot 2