You get warning messages similar to this in the Runner/Server Log view when you try to start your application from Squish:
Preload (71840001): Warning: Trying to load a different Qt library (C:\Qt\4.7.1\bin\QtCore4.dll) than the AUT uses (possibly C:\Program Files\MyApp\QtCore4.dll, C:\Program Files\MyApp\QtGui4.dll) - hooking into the AUT is likely to fail.
This typically means that you configured Squish to use the system's Qt libraries, for example, those in C:\Qt\4.7.1\bin. Squish should be using the Qt libraries used by the application; for example, if the application's Qt .dll files are in C:\Program Files\MyApp, then that is the folder that Squish should use when it needs the Qt libraries.
Run the Squish setup program (SQUISHDIR\setup) again, and configure it to use the Qt libraries that your application uses.