I think you have a missing (not a device standard ) font on the device.
For YOUR use (only): You
could just add the font file(s) to the
fonts folder on each Kobo.
or you could embed them in each book.
Calibre editor has a and item under
Tools: embed referenced fonts
(this may work if the font is found on your system: You say the font works for these books
)
If the books are for ANY KIND of distribution, there are legal hoops dictated by the font creator that MUST be followed. (Not all involve spending money ).
There is a slightly different method for when the above embed fails.
You must use the
File menu to add a font (use) to the book. it imports the files AND suggests (a clipboard) the @font to add *paste and edit) to the CSS
or (where used) HTML file .