Why are some emoji not displayed?
It depends on the NotoColorEmoji.ttf font file you have installed. If that file doesn't include a glyph for the Unicode code point you want, it can't show it. Here's an SO post about fonts and glyphs:
https://stackoverflow.com/questions/3582944/how-does-a-unicode-character-get-mapped-to-a-glyph-in-a-font

This reddit post talks about replacing your system's NotoColorEmoji.ttf font file with one that includes EmojiOne emojis:
https://www.reddit.com/r/Android/comments/3xezb9/emojione_on_android/