Hello Kai,
Are you using the WebViewer's Print button on the toolbar?
The browser's default print button will not work, because browsers do not print the contents of HTML5 canvases.
The WebViewer's Print button works by rendering the canvases as images, then appending it to the document body.
If you are using the WebViewer's Print button and it still doesn't work...
Please send us a document or sample project showing the problem.
Best Regards,
Kevin Kuo