There are four different 3-4 Expert Lessons (2007, 2010, 2013, 2016). I've tested all four of them using the latest version of Google Chrome running under Windows 10.
If this doesn't work for you there are two possibilities:
1. You are using an out-of-date or unsupported browser.
2. Your Internet connection is very slow or faulty.
3. Your browser has developed a fault.
We only support the latest version of the Google Chrome, Internet Explorer, Firefox, Edge and Safari browsers.
You will probably find that if you use a different browser (all five are free to download) you will solve the problem.
If you continue to have problems please advise the precise lesson you are trying to view (2007, 2010, 2013 or 2016), your browser type and version number and your operating system. i will then try to re-create the problem using the same environment.