Recently I bought an X99 motherboard and a Xeon E5-2690 v3, but the PC only outputs video with a 2666MHz RAM in a single slot; in the other slots it doesn’t output video.
Since you’re using the plural, I assume you have more than one RAM stick.
So, the first thing is to check if all the RAM sticks are good. Connect each of them to a slot that you know works and see if it boots. While you’re at it, test each of these RAM sticks with memtest86.
If all of them are good, now it’s time to consult the motherboard manual and check how the RAM sticks should be allocated in the slots. It’s not uncommon for a motherboard to require using slots 1 and 3 first, for example.
If you are following the manual and still not getting a boot, check the memory frequencies and whether the generation you bought is supported by the motherboard.
If everything is within the specifications, the problem is most likely hardware. Two possibilities you can check without much difficulty are whether there is no dirt trapped inside the memory slots and whether the slots are properly soldered. If they are, the problem may be somewhere else and you’ll need a deeper analysis and other components to isolate each part in order to make a more precise diagnosis.