As previously posted - try this as iOS6 does change your default store settings...
Make sure you are logging into the various stores with your correct Apple Id. For some reason, both my iPad & iPhone reset themselves to my original Apple Id., but my purchases were made against my "xxx@me.com" Apple Id. Oh joy! A real pain in the rear, but having signed in with my xxx@me.com Apple Id. on both devices, everything works again.
I just love the way that little things have been changed in the iOS6 upgrade... OK, it asked me whether I wanted to associate my puchases with my xxx@me.com Id., but I didn't know what the implication of saying no was!
Hope that works for you.
Dave.