Ed Cryer wrote:

There are four RAM slots; two blue, two grey. It came with one 8GB
module in a blue slot. I added a second 8GB module in the other blue
slot; all ok, memory checked by Windows 10.
So, I got two new modules exactly the same as the second one, and put
them in the remaining slots. And cram!
I took those two out and booted ok. I put the first new one in one grey
slot, and still cram! Same with just the second one in that slot.

This from the Mobo User Manual;
Memory configurations
You may install 1GB, 2GB, 4GB, and 8GB unbuffered ECC, non-ECC DDR3
DIMMs into the
DIMM sockets.
• Memory module with memory frequency higher than 2133 MHz and its
corresponding
timing or the loaded DRAM OC Profile is not the JEDEC memory standard. The
stability and compatibility of these memory modules depend on the CPU’s
capabilities
and other installed devices.
• You may install varying memory sizes in Channel A and Channel B.
The system maps
the total size of the lower-sized channel for the dual-channel
configuration. Any excess
memory from the higher-sized channel is then mapped for single-channel
operation.
• We recommend that you install the memory modules from the blue
slots for better
overclocking capability.
• Always install DIMMs with the same CAS latency. For optimal
compatibility, we
recommend that you install memory modules of the same version or date
code (D/C)
from the same vendor. Check with the retailer to get the correct mem

Help, help, help!

Ed

The BIOS reads the SPD (serial presence detect) of the memory modules to
get their timings. When there are mixed modules, the BIOS has to guess
which timings to use. The module with the fastest timings cannot be
used for the module with the slowest timings: you cannot run the module
faster than it is rated (unless you want to get into overclocking -- and
then hope a mismatched set of modules will reliably overclock the same).

https://en.wikipedia.org/wiki/Serial_presence_detect

It is possible the BIOS is configured to use SPD from the memory modules
or that you or someone used manual settings in the BIOS. Unless you are
into overclocking (doesn't look like it), the BIOS should be configured
to use SPD to set the memory timings.

Within the same bank, the slowest module gets used. Typically the first
bank's timings get used for all banks. When you put the same type
modules in the other banks, could be they are slower than the original
module. The BIOS probably uses the timings from the first module in the
first bank. If it is faster than your new ones then its shorter timings
get used on the slower modules. The result is the timings used for the
newer but slower modules are too short/tight. You need to move the
original module into the 2nd bank and put the slowest modules in the
first bank. Then the BIOS will used the timings for the slower modules
in the first bank, even for the faster module in the 2nd bank.

There is also the problem in architecture differences. Blocking can be
different in different modules. It has been too long since I got into
this but I remember some unscrupulous sellers were selling low-speed
memory at higher capacity that used a different architecture. The
result was addressing was affected differently across the mismatched
modules. You also must not mix ECC with non-ECC memory. We have no
idea if the original module is dual- or quad-channel and what you got
for the new modules, or what your mobo supports since you never
identified it.

Although the manual says you can install varying sizes of memory
modules, addressing is the same across all banks. If you add a large
and small module, you get the capacity in the large module only what the
small module can deliver. You waste you money on the larger module(s).

We have no idea what mobo you have. "Asus mobo" doesn't specify the
model number. We have no idea of the brand and model for each of the 3
new memory modules you bought or the brand and model of the 1 original
memory module to know if those modules are compatible. You don't just
slap any memory into the slots for it to work.

Since you bought 3 that were the same, you need to buy 1 more that is
the same to replace the original one. You do not need to buy matched
sets of memory modules but all should not only all have the same timings
but also the same architecture. Easiest is to buy all modules of the
same brand and model.