Usually it's down to the wrong language being selected.
If your Autocorrect is set to English UK and your language in your text frame is set to English USA - it can stop it working.
Where can I find the language settings for text frame?
Sorry, maybe that was a bit misleading.
It's in the Character Panel, there's a language selection.
If you're using styles it's best to change it in the Style Dialog options.
Thanks. That appears to have corrected the problem