Sometimes Ram sticks have the reason to stay at 1600mhz. why is beyond my understanding
what i do know if you can try to reset the CMOS. that way you give your BIOS/UEFI a little reset and can reset the ram speed of the ram sticks.
You can clear the CMOS by the following methods.
Some motherboards have what we call a Jumper. these have 3 pins on the motherboard. all you need to do is to replace the pins from 1 and 2 -> 2 and 3.
Hold it in place for around 15 seconds.
Place the pins back to 1 and 2.
if you cannot find the jumper, means you need to look into the motherboard specs. if it still isn't telling it where it is, then there is no jumper, but dont worry.
there should also be an CMOS battery on the motherboard. these are round, flat and very small on the motherboard.
remove this battery and leave it out the motherboard for a minute.
reseat the battery back onto the motherboard and startup your pc.
the startup can take a very little while longer, so don't panic.
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