[Gimp-developer] Cage Transform not completing

[Terri George <terrigeorgeauthor yahoo co uk>]

Hi
I've been searching online for the solution to the problem I'm having, but so far found nothing. I'm really 
hoping someone here can help me.
My problem is with Cage Transform. I create the cage and distort it, but when I click Enter to commit to the 
transform it doesn't save it and just reverts back to the original settings.
I want to create a 3D image of my books, but first I need to cover the front of the template png I have with 
white so I can then add my book covers to a white background ensuring the original background doesn't show 
through. So I added a white png as a layer. This is what I am trying to change the shape of.
I've attached 3 images to show what I mean.
Thank you so much to anyone who can help me work out where I'm going wrong and how to save my changes.
Cheers
Terri