when trying to use aldiko, it tells me that my tablet is not authorized.  What should I do?

I have previously authorized the tablet, and I can still read old DRM protected epubs I have purchased on it, but whenever I try to import a new epub I have pruchased, it keeps telling me the device is not yet authorized. I have de-authorized and re-authorized the product, with no effect. Customer Services were very unhelpful and directed me here.