If reseating RAM do not fix the issue it means the slots are lose or some oxidation present. Clean the RAM PCB with a soft cloth (microfiber) and Isopropil alchool. Alse can use a soft brush and same alchool for the ram slots.
Some DRAM modules PCB are thiner or thicker depending on the PCB used by manufacturer but standard should be 1.2mm. I have seen differences in PCB thicknes before causing memory slot contact issues.
I had a batch of Dell Optiplex with memory contact issues some time ago (10-12 years) but was covered by NBD warranty.
"Module packaging
DDR4 memory is supplied in 288-pin dual in-line memory modules (DIMMs), similar in size to 240-pin DDR3 DIMMs. DDR4 RAM modules feature pins that are spaced more closely at 0.85 mm compared to the 1.0 mm spacing in DDR3, allowing for a higher pin density within the same standard DIMM length of 133.35 mm (5¼ inches). The height of DDR4 modules is slightly increased to 31.25 mm (1.23 inches) from 30.35 mm (1.2 inches) to facilitate easier signal routing.
Additionally, the thickness of DDR4 modules has been increased to 1.2 mm from 1.0 mm to support more signal layers, enhancing overall performance and reliability.[53] DDR4 DIMM modules have a slightly curved edge connector so not all of the pins are engaged at the same time during module insertion, lowering the insertion force."
Source:
https://en.wikipedia.org/wiki/DDR4_SDRAM#:~:text=DDR4 memory is supplied in,133.35 mm (5¼ inches).
And i would say your ram modules are not the best option.
"RAM: Crucial RAM Kit da 16GB (2x8GB) DDR4 3200MHz CL22"
Me i'm using some decent ones like "
Kingston FURY Beast, 16GB DDR4, 3200MHz CL16, Dual Channel Kit"
