Download link not valid.

I have mp3 download links for sale using wp ecommerce. Starting a few weeks ago, customers are getting download links and when they’re clicked on, the link says it’s no longer valid. I don’t know what has changed. I’m sure this has been an issue others have posted but I must say, the search link given in the “before posting a new topic” link, really did nothing to help. When I get results on the getshopped website, the forum disappears as soon as I log in. I can’t even tell where the fourm help is!