Is the memory exact same, brand and spec's?  If not the same Dell brand as the original that can be the problem.  Mixing memory is a hit or miss and not a "Dell only" problem.  I have that problem on my self built using an ASUS ROG motherboard.  I have 16GB of Corsair memory and 16GB of GSkill memory that have the exact same specs.  Either the Corsair or GSkill work without problems.  With both brands installed I get a memory error and it will not boot.

However, clear the CMOS memory and see if that helps.  With the PC powered off, open the case and remove the 2032 coin cell battery on the motherboard for at least 10 minutes.  Reinstall, power on and see if this helps.

Did you try clearing BIOS while installing the second module?

1. Reboot and immediately press F2 to open BIOS setup
2. Copy all current BIOS settings to be safe
3. Power off, unplug
4. Press/hold power button for ~15 sec
5. Remove motherboard battery
6. Press/hold power button for ~30 sec
7. Install second RAM module
8. Reinstall the battery
9. Close up and see if it boots now

Unless you buy a matched pair from Crucial, they won't guarantee their module will run in dual-channel memory mode when paired with a module from some other manufacturer. 

I actually called Crucial because they list an 8gb module as an exact match for the existing 8gb in the Inspiron 5680. Their website says they guarantee their recommended modules are compatible, and the tech I spoke with knew I was intending to match the original 8gb (not buy a pair). In addition, he said I could buy it in dual or single rank (the latter was in my 5680) either from them or Newegg. The only reason I purchased the Hynix RAM on Amazon is because I saved $15 and that vendor also guaranteed compatibility. I assume Crucial would have stood behind their guarantee.

You're mixing apples and oranges. Dual rank means there are chips on both sides of the module. Single rank means the chips are only on one side. That's not the same as single- and dual-channel mode.

And being "compatible" means it will work which is all they can guarantee, not that it will run in dual channel mode with some other module, unless Dell happened to install the identical Micron RAM module in the PC. And more things go into RAM than just its number of GB and speed to make them identical which is what dual mode requires. 

(RAM running in dual channel mode is more efficient than RAM running in single channel mode...)

Thanks, but I wasn’t confusing rank with channel. I only mentioned rank because I figured I may as well match that, too, particularly since the RAM was available in both single and dual rank. Bottom line is that perhaps the safest way to ensure that one can match the existing RAM and that the two modules will run in dual channel mode is to buy exactly the same RAM—not Crucial or any other manufacturer’s.

For my 5680, I ran CPU-Z, wrote down all the specs for the original RAM (including manufacturer and part number) and then searched for same on Amazon and Newegg. I found a perfect match on the former site and the vendor assured me it would work fine. It did. If I’d opted for Crucial’s and it failed to run in dual channel mode, I would have returned it. The alternate approach of buying a matched pair of someone else’s RAM would also work, but 16gb total is all I wanted; i.e., I wanted to wrap this up for $50 rather than $100.

Normally, I’d recommend starting by checking Dell’s Parts department for a match. Although their stuff is more expensive, I’d trust it to be correct. For whatever reason, though, that section of their site for the 5680 has been “unavailable” for months.

The Ram slots are White and Black ejector ears. You don't populate 1 2 3 4  you do 1 then 3   then 2 then 4.

You cannot mix quad rank or single rank with dual rank nor can you use CL15 vs CL17 ram.

(PC4-19200) CL-17 – 288-pin 1.2V RAM – Non-ECC – Dual-Channel

You need to have pairs of ram in the banks.

You cannot mix speed and density or ranks.

Its not recommended you mix vendors aka hynix with samsung for example.

https://www.crucial.com/usa/en/inspiron-5680/CT12020316

DDR4 PC4-19200 • CL=17 • Dual Ranked • x8 based • Unbuffered • NON-ECC • DDR4-2400 • 1.2V • 1024Meg x 64 •

They must be LOW DENSITY and INTEL compatible.

Here are the configurations available on the 5680:

•8 GB DDR4 at 2400 MHz (1 x 8 GB)
• 8 GB DDR4 at 2666 MHz (1 x 8 GB)
•16 GB Dual Channel DDR4 at 2400 MHz (2 x 8 GB)
• 16 GB Dual Channel DDR4 at 2666 MHz (2 x 8 GB)
• 32 GB Dual Channel DDR4 at 2666 MHz (2 x 16 GB)
•64 GB Dual Channel DDR4 at 2666 MHz (4 x 16GB)

Here's the board layout

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@cbandtalk wrote:

Inspiron 5680 gaming desktop. Has 8 gig ddr4 2400 ram. Trying to upgrade to 16 with an additional 8gig same typestick.

Bios will not recognize the 2nd stick. However each stick on its own works fine?

Tried several times ruled out bad ram. Is there a bios setting im missing ?

Any suggestions appreciated.

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Getting back to the original question... How many RAM slots are on your motherboard? The specs say there are 4 slots, but the motherboard photo in the online 5680 Service Manual only shows 2 slots.  :Ick:

If you have 4 slots, which two are you using? They're pairs with different color retention clips for each pair...

EDIT: Speed types faster than I do...!

@zapt​ which ram slots did you use?  the motherboard markings for the slots from left to right are 3B-1W-4B-2W (B=black, W=white).  I used 1W & 2W.  The system recognizes all 32GB and boots, but I'm not sure if it is dual-channel or not.  The manual was no help.