I am having the exact same problem. I have been trying to figure this out for days. I got a refurb Galaxy S3 from Freedompop. From day 1 the phone has had problems with downloading certain things. For a while it I couldn't even download gmail attachments. Most apps non-Google or non-Samsung can't download. I've checked the SD card fix for Kitkat (I'm running 4.4.2). I have the "Unknown sources" checked (after all, I do have Amazon appstore installed). I have cleared the cache, uninstalled, reinstalled, cleared cache and data, taken the SD card out of the phone, and I have cleared the Dalvik cache on the phone. Nothing has worked so far. I would really appreciate finding some sort of answer to this. Amazon Appstore is the app that I'm using as an example, but I have lots of other apps that won't run properly as well. Handy Scanner can't save pictures taken, Amazon can't download, HumbleBundle can't download, either. However, I can text myself a link from the desktop, and install the apps that way. Amazon Music won't download songs, Olive Tree Bible Study can't download other bibles, oddly enough, Coursera CAN download videos, Rhapsody can stream, but won't download locally...I think that may be it. No matter what I look up, it seems to not work.
If someone can find an answer to this, I would REALLY be interested in hearing it. It'd be great to have a real working phone again.