Words not added to custom dictionary

Please can anyone advise - I have added multiple words by right-clicking, ‘add to dictionary’, however on a following day, the word is again underlined in red showing as mis-spelt. 

I have checked the dictionary directory and there are no files with a *.custdic extension which I understand are where these custom words should be contained.  

What can I do to successfully add words to the custom dictionary please?