rx 470 mining issue

Im using hiveos with nbminer (because teamred keeps giving me an dead card error). I have 4 rx470 4gb cards in that rig. One asus strix and three sapphire mining editions. By what i’ve read so far the cards should produce somewhere between 11-14mh/s but i’m getting like 6-9. One mining edition gets 10, the other two around 7-8 and the strix around 6-7. I’m running 1100 core and 2000 mem. Does anybody have any ideas why i don’t get the hashes i should?