I'm new to this tool, so I was having fun at putting mistakes in your sample text in English. I have changed "Paste your own text" into "Paste you're own text", but after I've clicked on the "Check Text" button, the mistake was not highlighted. Same with and click the "Check Text" button/ and click the "Cheque Text" button.
Hope this helps you as much as it helps me with German!
Kind regards (and sorry for possible mistakes, but I'm not a native speaker )