Are you using the screen or hooked process plugin?
Wells here's my theories.
Hooked Process Plugin:
I don't blame you if you don't get it working properly. It is quite glitchy. It can usually be resolved by clicking between the your game window and your podcaster window then restaring the stream.
A) Use the technique stated above.
B) You're using a source cam such as webmax instead of your video card source.
C) Your video card sucks ass.
D) Your processor sucks ass.
E) The aplication you're streaming doesn't use OpenGL or DirectX.
Screen Plugin:
If you can't get this to work then you have an IQ below -1.
A) You're using a source cam such as webmax instead of your video card source.
B) You're using 3D or 2D mix.
C) You have it set to webcam.
D) Your video card sucks ass.
E) Your processor sucks ass.