Not working 3 dice of RAM

Stood in a computer with 8GB, 2 4GB, I decided to buy another 4 gig to 12, but inserting in the slot the computer gave only a black screen. Bought the RAM coincides with existing, IBA it works I checked it on the working slots of RAM. M4A77T motherboard, 4 slots - two black, two blue. 8 gigs as I understand it do not accept it, only 4 gig, but then what's the problem? Why not start with 3 strips of RAM? By the method of permutation found out that all the same if the RAM insert in 1-4 then it works, but it is necessary to add 3 to the die somewhere then have a black screen, so basically 2 die was only 1-2 slots(black)