When using Digital Signage Web Template, the target web page is displayed within an iframe in the Digital Signage system.
There are two possible issues:
1) If the source web page is http and the digital sigange is https, it will not work. This is because https can be used inside of http, but http cannot be used inside https (for security reasons)
2) If the source web page has a HTTP header called X-Frame-Options with the SAMEORIGIN value the browser can only be display in a frame on the same origin as the page itself. Signage will not work in this case. The solution is to remove this from the web server (you cannot edit the page to remove this, it is a server setting).
However, for SignStick, this issues is not present because SignStick does not use iframe to display the web template.