I am running LO 126.96.36.199 on macOS 10.15 on my mid 2012 i7 16GB MacBook Pro. My language is en_US. Spell checking works on some document and not on others. On some documents it starts and stops. I have Tools | Languages | For All Text set to English. The styles all have English (USA) set. My LO | Preferences | Language Settings are all set to English (USA)
Since I am a poor typist, I really need the spell checker. Any suggestions on troubleshooting this?