2+2 = 0 RAM issue

[Looniper]

New system using Viper series DDR 3200 RAM (2x8 dual channel)
An identical system using the same ordered from the same supplier at the same time.
Both worked fine, but decided they needed 32 gigs (I tried to tell them)
One buys a 32gig dual channel kit.
The other takes their 16 gigs, and adds it to their system.
It won't POST.   It works fine with 2, regardless of which 2, but will not POST with 4 sticks.

It's in a MSI PRO B550M VC-WiFi Micro ATX AM4 motherboard

He's ready to get a new 32 gig kit, since they're not exactly steep.   I want to figure out why tit isn't working, just to know.
Anyone know what the issue is likely to be?
 

[Catzzye]

It's worded a bit confusingly but DDR4 is pretty flexible with it's compactiblity, worst case scenario it would clock down to the slowest RAM installed. It should work afaik.. Did they make double sure that when they were installing the RAM, both sides of the stick clicked in place? That's what happened to me once when I was filling all 4 slots.

[Blasty Blosty]

Have you tried resetting the BIOS?

[porina]

Running with 4 sticks often reduces the maximum working speed. 3200 isn't fast but not all CPUs will be stable with 4 sticks of it. Older Ryzen CPUs for example really struggled with it.

 

Was XMP on when running 2 sticks? Hopefully the mobo should detect the change in ram and use safe defaults, but in case it doesn't, put 2 sticks in to boot and make sure XMP is off when going to 4.

 

Give it more than one try, and give it some time too. First boot on a hardware change can take much longer than normal.

[Tegneren]

4 sticks is harder on the memory controller, that's why we recommend using only two in a dual channel system if possible. AMD Ryzen in general is harder to get to play nice with RAM than Intel systems.

Even if you buy two of the same RAM kits at the same time, you are not guaranteed they are exactly the same. Sometimes manufacturers switch out the chips they use on the RAM between production runs to something equivalent. It can be because of availability or just down to price, but in the end they are slightly different, which can be enough for a system to not recognize the together.

 

You should check the QVL for that motherboard if the RAM you are using is verified with four sticks or at all. 

Also updating the BIOS may in some cases help on memory support.