But, but it’s not really solved.
Seems that it is quite unstable to use fcitx on Ubuntu 18.04, even after all the settings. I still occasionally encounter the missing candidate problem. Thus I just uninstall fcitx* and use ibus-pinyin nowadays. Surprisingly, it works quite well and gives solid predictions, pretty amazing compared to what I remembered several years ago.
After upgrading to Ubuntu 18.04, my fcitx pinyin IME won’t show candidates.
rm -r ~/.config/fcifx
sudo apt-get purge fcitx*
sudo apt-get install fcitx-googlepinyin
This re-installation solution didn’t work.
The fix I later found is to install Chinese language support in Gnome setting, and choose fcitx as keyboard input source.
Settings-> Regions&Languages-> Manage Installed Languages-> Install/Remove Language -> Install Chinese -> Select fcitx as keyboard input method system.
Your IME is now willing to provide you with candidate characters.