Check to make sure the preview is not disabled:
Go to Edit -> WebAssist Preferences In the Preview Section, make sure the Disabel Preview option is not checked then click the OK button.
If the problem still persists, click the preview window in the extension, this should open a full size preview in your web browser, please let us know whether the full size preview opens or not.
also copy the adress that is in the browsers address bar to your reply for our information.