There are many reasons why Word 365 may not be spell checking your documents. Here are some of the most common reasons:
Autocorrect options turned off: The built-in autocorrect feature of Word 365 might be turned off. To check if it is enabled, open Word and go to “File” > “Options” > “Proofing” and make sure the “Automatically correct spelling and grammar as you type” option is checked.
Incorrect language settings: Word may be set to the wrong language. To make sure the language is set correctly, open Word and go to “File” > “Options” > “Language” and select the language you are typing in.
Lack of processing power: Word 365 requires a certain amount of processing power in order to run its spellchecker efficiently. If your computer is not powerful enough to handle it, then it may not be able to spellcheck your document.
Outdated Word version: It is also possible that your version of Word is outdated. To check if an update is available, open Word and go to “Help” > “Check for Updates”.
Third-party add-ins: Third-party add-ins, such as grammar and spell checkers, might be interfering with Word 365's spellchecker. To check if any add-ins are causing the issue, open Word and go to “File” > “Options” > “Add-ins” and select “COM Add-ins” in the “Manage” drop-down menu. Uncheck the add-ins you don’t need and then restart Word.
If you’ve tried all the steps and still cannot get your Word 365 spellcheck to work, you may need to contact Microsoft Support for further assistance.
I hope this information helps!