Have you noticed the sections of the book you've highlighted are no longer highlighted when you go back to them? Are the highlights saved in your Notebook a long string of characters with no spaces in between words?
If so, you've probably been using the ReadSpeaker tool to have the text spoken aloud recently. The text reader adds a sort of extra "layer" that causes the highlighter to misbehave. This is a known issue that we're working on resolving. In the meantime, you can work around the issue by following the below steps:
1. Refresh your page. This will turn off ReadSpeaker
2. Highlight the passages you would like to save.
3. Resume your use of ReadSpeaker (if desired)