microsoft office – Word is not spell checking?

I am making notes for uni on Word and recently Word has stopped automatially spell checking. After going into the Reveiw tab > Language > Set Proofing Language, I turned off “Do not check spelling or grammar” which had randomly turned on, however Word is now not picking up any mistakes that I made before I changed this back. This setting also changes back from time to time, so how can I stop this from changing back from unchecked to checked.

Furthermore, how can I make Word recognise the mistakes I made before so that I can correct them? I have found at least 2 that I made before I realised that this setting had checked itself, howver after I click on “Recheck Document” it still doesn’t identify the mistakes that I made before I changed the setting back to what it should be.