Occasionally some files will not download correctly (either partial file or simply a blank empty file). It's a problem with Instagram itself as other programs also have this issue (even downloading images directly off Instagram sometimes does this), re-downloading the item usually fixes the corrupted file.
As it is, the only way to re-download items is to completely remove them from 4kStogram and re-add them, so an option to re-download or verify files and re-download corrupted files automatically would work great.