Some websites can't be embedded as an iframe

[berb]

This is a known issue due to the X-FRAME-OPTIONS (https://developer.mozilla.org/en/The_X-FRAME-OPTIONS_response_header) in modern browsers. If web servers do not allow the frames to be displayed in external websites, they can't be used as part of the slide content.

Being a security feature, there is no way to "fix" this behavior. As a workaround for presentations, you can use an HTTP proxy and alter or remove X-FRAME-OPTIONS response headers. Note that this is a security threat and should be used only temporarily and with caution.