bios – Memory failure in DIMM2 Dell T3500

I'm having problems with my DELL T3500
It has a w5590 3.33GHz 4c / 8t
12Gb (3x4Gb) RAM DDR3 1333GHz
R7 250 2GB GDDR3
1 TB and 500 GB HDD

I receive the message when I turn on my computer and it starts to boot. Continuously I pressed F1 to continue, so I could use my PC, but now it shows in the Task Manager only 8Gb available.
How can i fix it? I already change the RAM memory places like 5 or 6 times. It always says DIMM2 regardless of what changes.