GPU help 2 gtx 1070

Hey guys. Iv been mining Zec for around 6 months using 1 gtx 1070 with out any problems. Iv just got myself another 1070 and a riser as the cards are too close to each other if attached to motherboard. I get very low sols when using the cards to mine for some reason and they both only draw around 100w of power each for some reason instead of around the 150/160 they should. I have a CX750M PSU and I didn't think that would have been a problem. any help much appreciated