Make sure that both TCP and UDP ports are forwarded to your Slingbox. Most of the people who can only get one device to support remote viewing but not another is usually not forwarding both transport protocols.
Another reason, could be that the mobile provider or hotspot provider is either using or blocking that port for their own nefarious reasons. In which case, you can attempt to bypass that by reassigning the port that the Slingbox uses to a more common port like 80 (HTTP) or 443 (HTTPS). Of course, if this is necessary then make sure you also change the port forwarding rules to reflect the new port number.
Finally, if you have a directory entry for a Slingbox that uses a direct IP address connection. Make sure that the public IP address is still current. Most people don't spend the extra money for a static IP address, so you should verify that the public IP address is current. Usually, you're better of just using the Finder ID through Sling Accounts to let it find the appropriate address for you.