Adding More RAM

I'm very new to this ... I've recently taken an old server and installed OMV, when I did it there was 12gb of RAM installed, I tried installing another 4gb stick today and I don't think it's seeing it, is that maybe because you can't add RAM after the initial install or is there somethng else I can do so it sees it?

I'm only saying this as I'm looking in diagnostics / system iand at the bottom .... Memeory Usage it showed a total 11.63 GiB which hasn't changed sine I added the extra RAM ... I've posted a screenshot below, any help appreciated

Cheers for the replies ... @mischka I haven't used anything Linux based until now so thought maybe Windows picked up if RAM had been added and maybe Linux would only do it on an install, I'll try changing the slots they're in, I also have one other stip so will try that as well.

@raulfg3 I knew that was a general principle, I was surprised that this machine has 4 slots but only 3 strips if 4gb RAM which was partially why I thught I'd buy some more so each slot had 4gb.

Just to clarify as well ... if I did have 16gb or working RAM, I am looking on the right place and the memory usage would then say approx 15.5?????