There are possible reasons:
A. Some of the fonts do not map the corresponding code point (aka. missing character)
B. Some fonts map the corresponding code point but there are not valid glyphes.
C. Some of the Chinese can only shows in Simplified style or in Traditional style. If there are simplified Chinese in a Traditional Chinese, it will not show or show as default glyphe. All you need to do it transfer it to the other style.
D. Some fonts have some problems of compatibility causing mistakes but it does not mean that the font is broken.
It happens because some fonts maps the corresponding code point of preview text but it only connects to default glyphe (usually as in box)
Click the pull-down list of "Preview", then you are able to choose any pre-installed preview text including 14 basic character strings and 28 pangrams.
The pre-installed basic character strings are Font Name, Latin Letters, Uppercase Letters, Lowercase Letters, Tone Letters, Symbols, Arabic Numerals, Arabic Alphabet, Greek Alphabet, Cyrillic Alphabet, Hiragana, Katakana, Hangul Numerals, Chinese Numerals.
The pre-installed pangrams are Bulgarian(Български), Icelandic(íslenska), Polish(Polski), Korean(한국어), Danish(Dansk), German(Deutsch), Russian(Русский), French(Français), Finnish(suomi), Gaelic(Gàidhlig), Dutch(Nederlands), Catalan(Català), Lithuanian(lietuvių kalba), Romanian(Română), Norwegian(Norsk), Portuguese(Português), Japanese(日本語), Swedish(Svenska), Serbian(Српски), Slovene(Slovenščina), Thai(ไทย), Spain(Español), Hebrew(עברית), Greek(Ελληνικά), Italian(Italiano), English, Simplified Chinese(简体中文), Traditional Chinese(繁體中文).
Tips: Pangram
Pangram is a kind of sentence contains all of the letters in alphabet. It is mainly used to test the effects of tested fonts.
Click the "Preview" button, choose the pre-installed preview texts or input customized preview text. If you want to adjust the parameters, you can choose the foreground and background colors. When you click "Reset" button, every customized parameter will be reset.
Tip: The default preview text in the "Result" page is the text you input in the "Stitch" page.