Select sample text - not all the texts work for my font

Hello! Can anybody help me understand what the problem can be with sample texts for my font. Most of the text are fine in my font, but in some texts some symbols are missing when I implement this particular text in my font (mostly the symbols near or in between / / (slashes). Here is the link to the screenshot with the example:

Do you have the glyphs like f_f in your font? If not, they can not be displayed. Whenever you see something beginning with a slash in the sample stings, this means the glyphname.

thank you, Mark! I had a ligature f_f, but now when I made a glyph with the name of f_f it worked with sample texts. But now I don’t understand what language uses glyph ff-(

Let me quote from here: »You may have noticed that fi and fl do not adhere to the naming convention we mentioned above. That’s because they are historical exceptions. These two glyph names were there long before OpenType.«
Basically meaning you are good to go if you always call your ligatures with the underscore. There is no specific language that uses /ff