Once in awhile, nvidia control panel will recognize both cards, and offer SLI.  Most of the time, recognizes only one card.
 
I tried swapping the cards, they both work fine.  I tried 2 different SLI cables, no change.
 
Windows 10, asus p9x79 deluxe, 64gb ram, 2x evga gtx 970s.
 
This has gone on forever, just been putting up with it, but now I'm ready to sell the cards individually and buy something else.  I'm just afraid I'll run into this same problem again (eg, mobo problem).

Welcome to the Forum karymcfadden
 
Are both Cards Running at x8 or x16 Gen2 or Gen3?
What CPU do you have?
Is your Bios Current?
Reset your Bios to Reset the PCIe Lanes in the Bios.
Is CSM Disabled in your Bios?
Do you have any Yellow ! in Device Manager?
What NVIDIA Driver are you Running?
Have you Tried Both SLI Forks?
What Power Supply Make and Model and Age?
Have you run DDU to complete a Clean NVIDIA Driver Install?
Is your Windows 10 Updated and Upgraded to the Anniversary Edition?
 

Thanks for the reply.
 
1) Both cards x16 gen 3.  I tried flipping all my pci's to gen 2, no difference.
2) Almost certainly not
3) 369.09.
 
Again, this isn't anything recent, has done this since day one, and continues to be intermittent thru a windows upgrade and who-knows-how-many nvidia driver updates.
 
I forgot to mention: Sisoft Sandra always shows 4 GTX 970's, which tells me each card has 2 gpus, but more tellingly, sandra is seeing both cards.
 
Till recently this has just been annoying, a waste of money, but these days, for modern games, not having the second card SLI is a deal breaker.
 

Seems you added some questions, which is good:
 
What CPU do you have?
i7-3960x

Reset your Bios to Reset the PCIe Lanes in the Bios.
Do a factory reset?  I could try that.  I'll see if I can backup the settings first, to a usb stick maybe.
 
Is CSM Disabled in your Bios?
Dunno, not familiar with this acronym.  I don't recall seeing this setting.

Do you have any Yellow ! in Device Manager?
No.

Have you Tried Both SLI Forks?
Yes, I tried the bridge across both, no difference.

What Power Supply Make and Model and Age?
Um....corsair something...like 1200...pretty beefy, not a crappy brand.  I'll try swapping the power connectors between the cards, maybe that's it.

Have you run DDU to complete a Clean NVIDIA Driver Install?
No, good suggestion tho, will try.

Is your Windows 10 Updated and Upgraded to the Anniversary Edition?
Keeps itself updated AFAIK.  I haven't done anything apart from that.  I dunno what AE is.
 
 

Have you Tried Both SLI Forks? Yes, I tried the bridge across both, no difference
A single Bridge or a 3-Way Bridge?
 
What are the Part Numbers of Each Card?
Do you have all 4 GPU Power Cables Plugged in?
 
Windows 10 Anniversary Edition is the Current Version of Windows 10. *You need this Version to run "Gears of War 4".
 
 
 

Update your motherboard bios.

A single Bridge or a 3-Way Bridge?
I tried a single bridge across the connectors closest to the back of the card, and then again across the other connectors.  I don't have a 3-way bridge.  I do have 2 bridges, tho, and could try running both at once, but I thought I'd better check around before trying that little stunt.
 
What are the Part Numbers of Each Card?
I'll pull the cards and find that for you.

Do you have all 4 GPU Power Cables Plugged in?
Yes.  I did remove and reinsert the cards, flipping them around to see if one was dead, since the monitors are only connected to one card, but they both work.  I did not try flipping the power connectors to see if any are not providing adequate current.  I'll do that next.
 
Windows 10 Anniversary Edition is the Current Version of Windows 10. *You need this Version to run "Gears of War 4".
Ah, I hadn't heard that.  What got me started on all this is I was thinking about trying Dishonored 2, but am reading many complain of pc performance problems with just 1 gtx 970.
 
Thanks for your suggestions; I'll give them a go tomorrow morning and let you know how that goes.

What are the Part Numbers of Each Card?
I'll pull the cards and find that for you. You do not need to do this. Go to My Products and they will show the Part Numbers
That is if you Registered your cards when you bought them.

As Sajin above said update your Bios on the Motherboard, is it the Current Version?
P9X79 DELUXE BIOS 4701 (Beta 4801) is the Current One https://www.asus.com/Motherboards/P9X79_DELUXE/HelpDesk_Download/
I would go with the 4701 2014/06/23

Well I'll be.  Did not know that.  Nice trick.
 
The site's only showing an old 8800 GTS I bought pre-korean-war, but I'm sure I registered the 970s after I bought em.  They included some video game as part of the purchase, or something.
 
Anyway, maybe I can pull that straight off the cards instead.