Why aren't the changes reflected, I made in the GUI?

Why aren't the changes reflected, I made in the GUI?

There is a vague thing going on.
I downloaded a java project with all the libraries it uses. I set it up in
netbeans. Now if I make any change in the GUI, it doesn't get reflected.
But if I make a jar file of the project by clean and build option, and run
it through the command prompt, the changes get reflected.
I have never faced this problem before ! What could be the reason for this
? The same thing happens if use the eclipse IDE.(The changes aren't
reflected)
What happens is that, the default project always runs. The project that I
downloaded.It doesn't include my modification.