Chromecast issues almost always stem from a Failing TCP within your router at the Slingbox location. When you're at the home location, you will need to Port Forward your router; just in case you are not familiar with the process, I'll link the Port Forwarding Guides for the Slingbox. You'll just need to choose your router make and model.
Now the port listed on the guide pages all reference the older model Slingboxes, so you will need to adjust that as follows if you have a Slingbox never than the Slingbox PRO-HD:
Slingbox M1/M2: 5301
Slingbox 350/500: 5201
I hope this helps
Sling Media Technical Support