RAM sticks behave strangely, what should I do?

Question

RAM

I sat working at a PC, at a certain moment it started to slow down, I saw that out of 8 gigs only 4 were used. Without hesitation, I googled the problem, checked the memory, it turned out that one bar did not work (I tried 0 reactions in both slots ((2 bars of 4GB each) On
the same day, I found the right bar in the store, I went and bought it (for 8 gigs).
I inserted it into the slot, everything worked, it became 12 gigs.
A few days passed, the same situation, the bar stopped working. For the sake of experiment, I decided to take the old bar , voila, it worked, but the new one flatly refuses, who ever faced this?