If they worked before and they don't now, one of them may have developed a fault.
Or maybe there's some crud on the contacts of either the ram sticks or the slot.
You should take a good close look at the ram slots and the ram contacts.
If there's any foreign material there, CAREFULLY lift it out.
For the ram, any dark spots or marks on the surface should be removed with a SOFT eraser.
Do not ever use an ink eraser. It'll remove the plating.
Use a pink eraser or a gum one.
If that doesn't fix it, then the stick may have developed a glitch.
Run a memory test from bios or Google memtest86.
It takes a while, but you have to let it run for at least one full round.
Good luck
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What is your system? Make and model.
Depending on what you have you may need to use a different set of slots (Banks Vs Channel).
crwdns2934271:0crwdnd2934271:0 Dan crwdne2934271:0