ga-p55m-ud2 64 bit wont boot with all four ram slots filled; will with 2 filled

[hellas93]

Hello,
 

My default ram map was

DDR3_1: One 2 GB stick

DDR3_2:empty at purchase

DDR3_3: One 2 GB stick

DDR3_4: empty at purchase

I tried to add two 2GB sticks in slots 2 and 4 (Crucial CT25664BA1339: which their memory scanner guarantees as compatible when I look up the product. The ram is DDR_3.  This PC is supposed to be able to handle up to 16GB of ram.

When I added these two sticks to slots DDR3_ 2 and_ 4, the PC would not boot; it would physically turn on, but would then begin beeping rapidly for about ten seconds, stop for a few, than start beeping again. When I removed both new pieces, the computer went back to normal. I have also tried to put in one of the new ram sticks at a time, but neither time did it work.

What could be wrong? are these pieces actually compatible?  Could there be a problem with the ram slots themselves?