Spell check intermittently not working

HI,

I haver a customer with an issue with outlook spellchecker. Basically sometimes it does not work. Even when there is a miss spelt word in it and you spell check it says complete no errors. This doesn't happen with all the emails and you can reply to the same message that had the issue and the next time its fine.

I have done the following

1. Created a new profile

2. Removed office and reinstalled

3. Added English US to the languages (main language is English UK)

4. Remove all traces of the outlook folder so it rebuilds.

Its office 365 business (2016).

Spell checker works fine in other office apps.

Any suggestions welcome.

Thanks

Noel