I use a light master and bold master to create 2 instances (book and medium), which both won’t display “a”. It’s the only character that does this… I checked every point on the drawing and everything is fine, checked it with the master compatibility and it was perfect. I really don’t know what to do with this one.
How are you testing your fonts? Are you using the Adobe Fonts folder?
This is possibly a font cache problem. Read this please:
can you send a screenshot of both masters with “Show Master Compatibility” active?