Hi
I've been sitting on Your site for 2 hours trying to solve it, but as far as I've noticed the problem exist when highlighter uses opacity. I have no idea why it's like that but I'm pretty sure I've seen something similar, so it makes me believe that some plugin is interfering with this function. Please try to unpublish plugin after plugin, module after modul till it starts to work.
I've noticed that "Horizontal stripe" is working so You can use it now and test the site localy in looking for a guilty extension.