LArge problem - Cannot unload sprite

I am having a problem unloading a sprite that generates thumbnails on the stage. The premise is that I have a few buttons of categories, one you click on the category, the thumbnails appear on the main screen.
This no problem once, but every other time, the new thumbnails load ontop of the others. I cant seem to get rid of the first sprite so that I can load the next.

I have built the thumbnail generation as a function, for which I have to pass the url of the xml file with the onclick command of category button.

I have tried a removeChild command with the other button onclick events but I get an error saying that the child returns as null. It seems to only acknowledge the sprite whilst it is buillding it…

PLease help, I am losing my hair! If you need more info please let me know.