I have many fonts that show up as Installed that do not appear in Libreoffice 6.0.3.2. I have just upgraded to Ubuntu 18.04, and they appeared in 16.04.
Which version of LibO was in Ubuntu 16.04? In version 5.x, LibO dropped support for postscript fonts: if the fonts you cannot see are postscript, you should look for an OpenType alternative.
5.x was the Ubuntu 16.04 version, and these are not postscript fonts.
Which are those fonts that do not work? Do they work in other applications?