Jauni do you see a browser error where the iframe should be ? A probable cause is that the browser is refusing to load an iframe due to the forum and iframe origins not being compatible, or because some headers are returned by one site or another. If your forum is HTTPS and the iframe HTTP it's already a bad start, I doubt any browser will load that.
If you don't see an error in the frame directly in the browser, there might still be an error visible in the console of the browser developer tools.
If you have a public demo page where we get see the issue, please share the URL.