It's been happening to me too quite a bit too. Try copying what you put in, refresh the page, the paste your code and see if that works. Sometimes it takes me several refreshes for it to take
actually this was from another post and worked for me:
- Go to previous lessson.
- Set screen zoom sufficiently high so that the preview window disappears from view. Upwards of 200% scale.
- Advance to the next lesson.