If on the performance tab (very left side in the list CPU, Memory, Disk, ...) it shows eg 8GB/15.9GB then its seeing all 16GB. UI's can be wrong and everything can be right so you should cross check that in a 3rd party application and in the BIOS as well.
Also, Task Manager may not show all the RAM being used in a way that you can add it up. It won't necessarily show it all or in a way you are expecting.
Windows RAM use is a lot more complicated than the old days. Users don't easily understand what its doing anymore on top of those possibilities.