This isn’t necessarily a Dorico-specific question, but it does happen in Dorico.
I’ve installed the MusGlyphs and Metrico fonts and am using them in a project, and everything works as it should.
When I close Dorico, shut down the computer, restart it and then reopen the project, Dorico displays a message saying that these fonts are missing and may be replaced with other fonts. I quit Dorico again and reinstall the fonts. After that, it works without any problems. But as soon as I quit Dorico, the whole thing starts all over again and the special notation writings are wrong displayed.
6 point what? There have been some micro changes in font handling. That is the correct install procedure according to the Documentation at Notation Central. I assume you rebooted after installing the font?
The other critical thing (for anyone reading this) is to check whether the dialog is telling you that “the font” is missing – or whether a particular style of the font is missing, e.g. SemiBold or Condensed. That style may not exist, but a Paragraph Style or Font Style is calling for “the default font” in style which it doesn’t have.
If Andro’s post was the solution, then he should receive the solution check, not just as a courtesy to him but also because the solution check attaches the helpful post to the original question at the top of the thread to save time for others who come to this thread for a solution.