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Hmmm, there was a problem with installing many apps from the Play Store related to encryption on 4.1 devices, but I thought it was resolved a year or so ago. So I have heard of a similar problem, but I don’t know what would be causing you to experience it now. The two things I can suggest are to install a file manager and see if you can find the files, so you at least know whether they are actually being downloaded. The second thing would be to sign up for an account at android central and visit the Galaxy Tab forums.
I don’t have a Tab, so I don’t specifically know that forum, but most of the forums are pretty active, and there are usually some knowledgeable people around, so someone may be able to help. Sorry I can’t offer anything more specific, but I hope you get it figured out. Good luck!