Less than half of my memory is available

I've been trying to track down an issue with my computer randomly resetting, and while checking for problems in my memory, I noticed that while I have 32 GB installed in my machine, in my system properties it says I have 32 GB installed but only 16 GB usable.
I don't recall it saying this before, but it has been a long time since I've bothered to look if I have all my RAM.

I was looking around online, and I see people asking about this phenomenon, and being told that the hardware is reserving portions of the memory. That may be fine and dandy if I only lost a couple gigs, but this is literally HALF of the memory on my machine.

What gives?

But now I've come across something that is even more bothersome, but may give a clue as to what is going on.
I was testing my RAM to see if there was a hardware issue leading to my random resets. (I ran the memory diagnostic twice and it came up clean, but maybe its hardware anyway.) I pulled out half of my RAM to just 16 GB. I noticed that the system no longer said that only part of my memory was usable. The next day, I swapped out the DIMMS again, this time only using the RAM I pulled out the day before.
NOW my system says I have 16 GB installed BUT only 7.96 is usable. Now I have less than half of my memory available!

Now I want to say that this means I clearly have a physical problem with one or more of my RAM DIMMS, but the math doesn't add up.
If there was a straight-up problem with the memory, wouldn't the same amount be unusable? Why would I lose 16 GB when all four are installed, but only lose 8.04 when just two are installed? And why would the memory diagnostic not find any problems?

Other information:
-All of my DIMMS are the exact same brand and model.
-When I tried running the RAM with just 16 GB, in both configurations I used the same slots on my motherboard. The manual states that if I use only two slots, those are the two slots I should use.
-I tried re-seating all of my RAM last week, trying to solve a completely different issue (which is now resolved.) Before then, I had never touched my RAM since I built the computer a couple years back. It was after doing this that I began to have the problem of the computer randomly resetting.
-It was after re-seating my RAM that I noticed I had only 16 GB usable.
-When I re-seated the RAM I intentionally put them all back in different slots, just to maximize the effect of re-seating.

So if I'm to understand this right. No matter what slot you use the RAM only shows half the amount?

What did you do prior to the RAM having this issue?

Is the XMP profile set in BIOS?

What tool did you use to test the RAM? I use Memtest x86+

My verdict thus far is either faulty RAM sticks. Meaning you have to test one at a time. Or the motherboard.

Finally, it may not matter, but is your RAM listed with the MOBO's QVL list? I always buy my RAM that's tested to be compatible with my motherboard. You may not have to follow that, but I do anyway to guard against any issues and what not and I just HATE doing returns.

How many RAM sticks you have?
What is the brand and model of your MoBo?

I've seen this happening on MoBo with memory that wasn't qualified (chipset).

I've been testing the sticks individually. I found one particular one that, if it is the only one in there, the computer will not boot with. I've left it out and I no longer have any statements from the system that only so much of the memory is usable.
So I guess that one stick is the problem.
Why it can reduce the performance of the rest of the RAM is beyond me.

In answer to the other questions, I am running an ASRock 970. This is the RAM I was using. I had four sticks of 8 GB each for 32 GB total. Now I'm just running 3 with 24 GB total.
The memory diagnostic tool I was using was the one that comes with windows.

You've found the problem -- replace that one RAM stick, and you'll be back in business with 32 GB of RAM. Or just go with 24 GB.

As for why it caused other memory not to be available, perhaps it caused a conflict with that memory. At any rate, you have definitely identified the cause of the problem -- that one RAM stick.

In a dual channel, bytes are split and sent half to a stick and half to the other, similar to a RAID 0.
I think that one bad memory stick can make the other (in dual channel) not to work.