Sometimes, furigana is not shown
For some words, furigana is not shown at all. I think this is when the headword has kanji and katakana, eg. 国際連合カンボジア暫定統治機構【こくさいれんごうかんぼじあざんていとうちきこう】. The issue being that the app can't match the katakana part with the kanji pronunciation at all. A solution would probably be to convert katakana to kanji on both side before matching parts.