about the use of avalon streaming interface

Really it sounds to me like in your simulation you've got something not quite right.

Are you sure the video into the clocked video input is active?

Are you sure you've asserted the locked signal?

Are you sure the timing of your external video generator is correct. The clocked video input will discard it if it cannot find some consistency.

Did you create the clocked video input with an avalon control port? If so you've got to write to the control port before the clocked video input will start outputting video.

Jake