ram slots used 2 of 4 - I have 16GB RAM but it's only showing 8GB. 888slot net

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.

In my task manager "Memory" tab under performance, It tells me that 4 RAM slots are available. But when I open the cover of my laptop, there are only 2 RAM slots visible.

a quad channel motherboard with 8 slots (X99 i take it) means that it requires its memory to be used in sets of 4, since you have 8 slots on the motherboard you can put 2 sets of 4 sticks in there.