You can try to add a short delay in your script since you have to wait for the object to be visible the QA Wizard Pro script maybe running to fast.
You could use the Delay(2000) to wait 2 seconds and then work on the item.
The question is does the dropdown appear in your application repository? If it does that is great and most likely you just need to slow things down to wait for this object to appear.
The other option is using the WaitForExists statement.. you put in the maximum time to wait for the item / object
bolFound = Window("Login").Button("btLogin").WaitForExists(5000)