I am sorry to hear that one of your iPad's is unable to connect to your Slingbox. There are a few questions I have for you to better understand what might be causing this issue. Are both of your iPads up to date and running the latest iOS version? Is the SlingPlayer application up to date? Are you connected to the Internet the same way on both devices'? If you're still having this issue please respond with the answers to the questions asked.
One thing that I would like for you to try is to uninstall and reinstall the SlingPlayer application. You will not be charged again for the app. Also, please see the link below for iPad compatibility.
If you have any more questions please feel free to reply.
Sling Media Moderation Team