在VS2005中调试Webservice

I am trying to debug a webservice by steping into a web method called
from a windows form project. The strange thing is that sometimes it
works but often it does not. When it does not work I get the following
error when trying to step into a web method:

Unable to automatically step into the server. The remote procedure
could not be debugged. This usually indicaes that debugging has not
been enabled on the server. See help for information.

Here is how my project looks like:

1) project: WinForm project which has a button that activates a web
method.
2) project: Web service which is consumed in project 1.

Solution -> Set Startup Project... -> Common Properties -> Startup
Project:
Multiple startup projects (注意：设置两个项目同时为启动项目)
1) entry: WebServiceProject (Action = Start)
2) entry: WinForm project (Action = Start)

Web.config is set to:
<compilation debug="true"/>
<authentication mode="Windows"/>

(注意：WebService项目的启动属性设置：等待外部程序请求再启动 )
WebService project -> Property Pages -> Start Options
Don't open a page. Wait for a request from an external
appplication.