XS728T Connection Issues at 10G

Hi, I have two new XS728T switches sat on a bench and am unable to get a 10G Link with a 35M cat6a cable however short cables link at 10G without any problems. I can go into port configuration and change from auto negotiate to 1G full duplex and I will get a successful link which will remain even if I cycle the power to either switch. Occasionally if I go from 1G full duplex back to auto then a 10G link will be established but if I cycle power at that point on either switch I lose the link altogether. Both switches are running the latest firmware