This is the output of :
$ sudo free -h
total used free shared buff/cache available
Mem: 5.8Gi 998Mi 4.1Gi 65Mi 748Mi 4.5Gi
Swap: 0B 0B 0B

This shows that my PC detects 5.8 gigs of RAM. However, I have two sticks of 4 GB installed.
This is the output of
$ sudo dmidecode --type 17

_` # dmidecode 3.2
Getting SMBIOS data from sysfs.
SMBIOS 3.1.1 present.

Handle 0x002A, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x0023
Error Information Handle: 0x0029
Total Width: 64 bits
Data Width: 64 bits
Size: 4096 MB
Form Factor: DIMM
Set: None
Locator: DIMM_A1
Bank Locator: BANK 0
Type: DDR4
Type Detail: Synchronous Unbuffered (Unregistered)
Speed: 2400 MT/s
Manufacturer: Samsung
Serial Number: 03230E2A
Asset Tag: Not Specified
Part Number: M378A5244CB0-CRC

Rank: 1
Configured Memory Speed: 1200 MT/s
Minimum Voltage: 1.2 V
Maximum Voltage: 1.2 V
Configured Voltage: 1.2 V

Handle 0x002D, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x0023
Error Information Handle: 0x002C
Total Width: 64 bits
Data Width: 64 bits
Size: 4096 MB
Form Factor: DIMM
Set: None
Locator: DIMM_B1
Bank Locator: BANK 1
Type: DDR4
Type Detail: Synchronous Unbuffered (Unregistered)
Speed: 2400 MT/s
Manufacturer: Samsung
Serial Number: 03230E2A
Asset Tag: Not Specified
Part Number: M378A5244CB0-CRC

Rank: 1
Configured Memory Speed: 1200 MT/s
Minimum Voltage: 1.2 V
Maximum Voltage: 1.2 V
Configured Voltage: 1.2 V_
`

Hence, Solus can detect both the sticks. But, then, why are not showing up in $ free -h

Well if you have 2x 4gb sticks and its detecting 5.8gb of usable memory it has to be detecting both sticks.

Some things to consider:

  1. Some RAM reserved / used by the kernel may not be reported as available RAM depending on the program used to read system information.

  2. If you have iGPU a certain amount of system memory is reserved for the GPU and thus not reported as system RAM.

  3. Difference in units being reported MiB vs MB / GB vs GiB

My system 3900X with 16GB RAM (no iGPU)

free -h
              total        used        free      shared  buff/cache   available
Mem:           15Gi       3.8Gi       3.1Gi       345Mi       8.6Gi        11Gi
free --giga
             total        used        free      shared  buff/cache   available
Mem:            16           4           3           0           9          11

If you have a iGPU you may be able to reduce the amount of reserved RAM in the BIOS / UEFI.

Further reading:
https://unix.stackexchange.com/questions/562720/system-monitor-in-ubuntu-18-shows-only-5-8-gb-ram-out-of-8-gb-installed

    devmrfitz
    So then #2 applies. The integrated graphics chip on the 2200G has reserved some system RAM for its dedicated usage and as such that RAM is not reported, its being treated differently. It is working as intended, nothing is "missing" BIOS / UEFI is probably set to dedicate 2GB to it, which from a quick google search is what is recommended.