I have about 5000+ photos on my iPhone 5s (iOS 8.3) but nearly all of them do not have the Favorites heart button available to favorite them in Photos. What criteria make this available for a photo? I'll add that I did not yet sign up for iCloud photos with the new Mac Photos app and I sync my photos directly with a lighting cord between the Mac and the iPhone, if any of that make a difference. (I can favorite photos on the Mac but that does not seem to sync over.) Thanks.