In my experience, the auto spell checker built into the story editor comes and goes. I can work for hours without it, then I save the page and all the red lines appear. (My stories have lots of made-up words, and therefore lots of red lines, so it's obvious when it is and isn't working. As of this moment, it is not working.)
I have yet to detect any rhyme or reason for this. The best theory I can come up with is that the spell-checker is a union member, and therefore won't work more than the minimum expectation.