Good afternoon!

My computer doesn't see the memory in the ram slot.
DIM1,2,3 currently has 3x4GB of ram and only sees 1.

So far I have used several RAMs, I thought I would buy a RAM and see how it is, I took one out and then put it back. The machine beeps only when there is a black screen and a ram error. So far, 5 pieces of RAM worked perfectly. There are only 3 in it right now, but you only saw 1.

I checked the CPU socket, no problem.
I cleaned the RAM socket.
The basic memory test shows that there is a piece of ram in the motherboard.
The bios also only sees one ram.
I tried several types of memory, even then it only recognized 3 figures.
I have already removed the bios element several times.
I tried the rams on other machines, it worked fine on another
The computer did not see 1333mhz 1.5v unbuffered memory. RAM is included in 1,2,4,5,6. Only in the 3 slots

Type: Dell Precision T3500
CPU: W3670
RAM: Kingston 9965432-082.A00LF
Bios A17

So far, it works perfectly with 5 pieces of ram.
I know the ram is not 1333mhz. I bought it with this RAM a few years ago.
Can someone help me?