Google's spell check seems to adapt based on your spellings. If you misspell several words when you start typing it decides you don't care about spelling, shifts to a less aggressive spell correcting protocol so that it won't hurt your feelings.
The program also seems to believe you would never paste a large chunk of text into your document that was full of errors.
So, if you paste or duplicate a document it trusts you are happy with the spelling and turns off spell check.
If this happens here is the first thing you should check:
If spelling is not working then select English(United States) and prepare your ego for a hit. It should start pointing out your horrible spelling skills right away.
Thank you for reading.