Problems with Upload.ChangeListener

I have a problem with Upload.ChangeListener. I am using it in a Form. I have in the form an upload component, the upload.changeListener, the method filenameChanged(ChangeEvent e).
When I enter in the form I select a file, the method filenameChanged is activated (it tell me a file was selected) and then I can upload the file selected. Finally I clean the components (put the values in “”).
But when I try to do it the same thing with a second file, the method filenameChanged isn’t activated, so I cannot detected if the file has been selected by the component and I can’t upload it.
Please help with this. It is important to activate the method.
Thanks a lot.