I found that if there are multiple panels in the scene, the second panel's widget could not be shown in unity "Scene" window. (But in "Game" window, it works fine.....)
I have used "align with view to selected" with camera, but the problem still exists. If I move view in the "Scene" window, occasionally it could be shown......

(Example 7 could reproduce this problem)
I haven't been able to find a similar problem in google and forum.
I use Unity 3.5.6f4 and NGUI 2.2.6 and I have snapshot a image for this problem.
Thanks for the help!