I'm getting the same problems as you. My smooth streaming videos are playing constantly on very low bit rates. However the BigBuckBunny plays fine from my server.
I contected Tim Heuer from Silverlight and he said my videos work ok. He even emailed back a graph showing the bit rate simulator playing the high quality stream.
I still haven't been able to solve the problem, as I can only get the low bit rate output from my own encoded videos (using Expression Encoder 3).
Maybe its my network/broadband setup? Have you tried playing back your videos from a different location i.e. internet cafe?
Here's my example, can you get a high bit rate stream from it?
http://67.15.149.7/Video2
(to display the bit rate graph, click on the button in the bottom right hand corner)