I use 6x MSI RX 580 8G perfectly for about 1 month in Biostar 12gpu motherboard.
Last week I bought a Sapphire RX 550 4G, it also worked perftectly as 7th GPU.
Then I bought 5x Gigabyte RX 550 2G (two is factory OC).
These cards didn't work with XMR mining in any of miners (Claymore, Stak, Sgminer...)
I bought a new motherboard (MSI H270-A PRO for with 7gpu).
They didn't work there also. There is no hardware problem including risers.
Simply the operating system don't recognize these cards.
I can see them in the Bios and also in Simplemining.net but i get different errors for different algorithms.
Claymore: No AMD OPENCLGPUs found
Stak: UNKNOWN_ERROR when calling clGetPlatformIDs for number of platforms

What is the problem? Thank you in advance for all the support that i will get.