If you have 2 apple Id's -1 for iTunes and 1 for iCloud like I do, it will not allow you to Family Share. Your ITunes ID MUST be your iCloud ID.
I'd say that's a major problem because a lot of Apple users have this kind of setup as a result of being iTunes customers before the iCloud/Mobile Me days. But Apple has done nothing to address this over the years, and they probably won't now unfortunately.