First, try changing to Google Chrome. Chrome has a built-in Flash plug-in that may resolve this problem for you. You can download it at https://www.google.com/chrome/
If you are using Chrome and are still unable to review the practice response, your firewall is probably preventing the response from playing back. If you are on a university, hotel or public network, the strong firewalls can interfere with playback. Even on a home network, firewalls often interfere with video playback as well.
Typically, video playback issues do not interfere with the recording in any way. As long as you can see yourself and the green audio bar moving up and down the side during the setup and practice step, your video questionnaire should record properly. If you are uncomfortable completing the questionnaire without reviewing your practice response, try switching over to a different network.
Please keep in mind that you cannot review the recordings of the actual video responses. The only video that can be played back is your answer to the practice question on the practice step.