The first place I would look is at the stuff on your network. My first thought would be that maybe you have multiple routers. if - say - your modem is also a router and you have your motorola router plugged into that, you are in a double-nat situation and setting up port forwarding on the motorola won't do you any good, if that makes sense.
I think there are some articles on the slingbox site about multiple routers, if that does turn out to be the issue. but if that isn't the problem, let us know more info about how your network is set up.