Update: This should be fixed now in 1.4.3, at least until Google Docs changes again.

Thank you very much!
I'm sorry about the trouble with Google Docs. Google have been gradually switching to a new "canvas" mode where the text in Google Docs is not available to extensions to read.
I hope to add a feature to 10ten to force Google Docs to use the old mode. Hopefully that will be added in a week or two.
Until then, sometimes if you load the same document in a new tab, you will get the old mode and then 10ten will work again.
Sorry about the inconvenience!