I just figured something out and I post this just in case people search the forum because they have the same problem.
Quite often I tried to add some symbols and they didn’t show up. So I displayed the symbols library and these symbols didn’t show up there as well. All other symbols displayed perfectly fine.
I’ve pressed ctrl + k to hide all markup.
So if you add a symbol with markup while in that state, the symbol will be added but doesn’t show up.
And if you display your symbols library these symbols will be hidden in the library as well.
That was very confusing for me until I figured it out.