Today I tried to upgrade the RAM of a T310 server (one X3440 processor) using 4x8Gb memory modules 

The mainboard has 6 slots in a row. (The 1st and the 4th slot have white clips).

So, I've installed 2 dimms on the 1st and 2nd slot, and the other 2 ones on the 4th and 5th slot (starting from the top)

When I start the server I have a message : Memory is configured but not usable

The Rams are : 8GB 2Rx4 PC3L 10600R-9-10-E1 

What's is the pb ? thank you in advance for your help  

BR