why put ram in slots 4 and 2 - I have 16GB RAM but it's only showing 8GB. 888 casino login slots

So I have 16GB RAM. After upgrading to Windows 10 however it says I only have 8GB. When I look in task manager it seems to know I have 16GB but for some reason only using 8Gb of it and for slots used it says 2 of 4 when I'm using all 4 RAM slots. I'm not techy at all but if somebody could explain to me why this is it would be appreciated. If it is something like just a broken RAM stick then that's easy to fix but I don't want to go out and buy a bunch of new RAM if it's a different issue.

I use DIMM1-3 and 4 slots booted but not seen slot 1 RAM. How to fix these two problem? ... i tried put 2 sticks into slot 1 and 3, 1 and 2, 1 and 4, 3 and 2, 3 and 4.

When I put one new RAM in the STD slot and an old RAM in the RVS slot only then it works.