connection issues between a catcher and a SlingBox could be related to the way the router its communicating with the server; in cases where the catcher wont connect but a PC or MAC player will do the most common troubleshooting that its reccomended its to setup a manual rule on your router; the how to can be found here.
Try setting up a Port Forward rule manually in the router settings and this should allow the Sling Catcher to view the slingbox trough the network.
Hope this info helps you out on this one.
Make sure your port forwarding is set properly on your router. If you are still having issues, your internet connection may be too slow.
I had an issue with one of my Pro-HD's and my SlingCatcher when I was using DSL at the Pro-HD source. Once I switched to Cable internet and the higher upload speeds I no longer had any problems.