Report Header invisible in Reportviewer but visible in pdf and print layout.

I met a wired problem when using SSRS in vs2008.

In the report page header, a textbox call a reportItem which connected to DB to retrive data in page body. The problem is the textbox shows blank in page header with no data, but when it's exported to pdf, everything is ok.

It really puzzled me for several days. And the solution is :

Change the called report item position---Drag the report item you called above the body's tablix, don't let the tablix totally cover it. Then it works well.  

May be it's MS's defect.