AMD A8-3870 with crossfire Ati 6670 DDR5 low profile

[Piplfox]

Hi I have Asrock motherboard A75M-ITX with A8-3870 and Sapphire 6670 1GB DDR5. My problem is that I cant meke them work in dual grafic mode (crossfire). I ve tried with XFX 6670 1GB DDR3 and it works perfectly. So my question is there a problem with compability or there is some kind of special procedure how to make them work tougether. Thanks in advance.