Spelling Correction Not Working OSX

I have a problem similar to the user who asked this question:

Can I disable auto-correction of a single word on OS X?

I have tried the suggested methods to try and get OS X to stop replacing 'CDs' with 'Cds' whenever I'm working in Outlook - I have told it to 'learn' the word, added the word CDs to my local dictionary and added it in System Preferences > Keyboard > Text.

I now have the ridiculous situation where 'Cds' is replaced with 'CDs' then corrected back to 'Cds' again as I type.

It is driving me crazy. Help.