不能在PIX里DEBUG Pixel Shader？ Can't Debug Pixel Shader in PIX

DONinLIVE

为什么有时候我不能在PIX里DEBUG Pixel Shader呢？
Why sometimes I can't debug Pixel Shader in PIX ?

我在SURFACE面板上右键点击“Debug this Pixel”，有时候我只得到一个没有“Debug Pixel”选项的结果：
I right-clicked on the surface and select "Debug this Pixel", sometimes I got only one result without "Debug Pixel" option in the Debugger Panel:

------------------------------
Final framebuffer value
------------------------------


但是有时候我又可以正常Debug，出现以下画面：
And sometimes I got this, with which I can continue debugging:

----------------------------------------------------------------------------
Event 1226: IDirect3DDevice9:rawIndexedPrimitive(D3DPT_TRIANGLELIST, 0, 0, 24, 0, 12)Primitive 12 of 12

Vertex Shader:0x030527E8
Debug Vertex 0
Debug Vertex 1
Debug Vertex 2

Pixel Shader:0x03052778
Debug Pixel (230, 186)
-----------------------------------------------------------------------------


为什么啊？
Why?

我已经用D3DXSHADER_DEBUG 设置好了我的程序
I've set D3DXSHADER_DEBUG in my App,
而且也在“DirectX Control Panel”这样设置好了：
and I've set the "DirectX Control Panel" as:

1. Use Debug Version
2. Enable Sahder Debugging
3. Debug Output Level=More

还有的是，有时候就连Vertex Shader也不能正常Debug，跳出一个错误消息框出来。
What's more, Vertex Shader debugging doesn't work neither with a error message box prompted.