help: "incomingConnection" does not respond in release mode

I use VS2005+Qt4.3.1.
When I build my project attached in debug mode, the program run property. That is

when I click Start button, and then telnet the machine, for example, telnet 127.0.0.1 4000,
then <<<<<<<MainServer::incomingConnection>>>>> (in file MainServer.cpp) will show.

But When I build the project in release mode, it seem that incomingConnection doesnot

respond.
Who can tell me why?

What happens if you show a message box instead of using qDebug()?