I know that people have had to go through some workarounds at times to get 6 GPUs working on Windows 7. We are mining with Windows 10 Pro on all of our rigs and I can confirm that particular motherboard should be plug and play with 6 GPUs and Windows 10 Pro, using 16.9.1 drivers or 16.12.2 drivers from AMD.

If you haven't done so already, download DDU and run it in safe mode to uninstall your GPU drivers and then install one of the two AMD drivers that I mentioned above. If that doesn't work (and nobody with Windows 7 exp comments) consider buying a Windows 10 key for $15 and install that.