There's plenty of similar questions on the internet about this board, just google "GA-970A-DS3P mining" and you'll find identical topics from people not being able to run more than 3 cards. Check whether you have the latest BIOS, and if not, update to the latest version. Also check the revision of the board you have, there seems to be multiple revisions of this model. Reinstalling windows helped some people, not sure why though but might be worth a shot.
I did a google search and saw that others are having the same issues as me however there doesn't seem to be any concrete solutions for this particular motherboard. I have rev 2.1 (the latest version), updated to the latest bios, and enabled IOMMU. Nothing seems to be working unfortunately. There are no advanced settings to tweak PCI-E settings either as far as I can tell.
Am I better off returning the motherboard and getting a new one. If so, what are some good options that are compatible with my FX 6300?