Hi Filio-Maria, I'm going to try and help with your query. I have some clarifying questions to ask and some tips below.
Is the issue you are experiencing in regards to Word not auto checking/highlighting whilst you work with a document? Or are you having issues manually instigating a check using the Review tab?
Is the issue related to Office 365 online or with the Word application on desktop?
If the issue is Office 365 related you can find settings for Spelling and Grammar by heading to the Review tab and selecting the Editor button via the arrow (shown below). From there you can access Editor settings.

If the issue is Word Desktop related you can check your grammar settings using (File>Options>Proofing) to see if any of the settings have been changed.
I hope this issue is resolved for you soon.
Kind regards
Andrew