This is happening to me, too. The "main" competition will consistently give out rewards but the "minor" competition gives out nothing. I get the items back at the end of the fair but no ribbons given, not even a participation ribbon. So my chickens, cows, etc, will all take home some sort of prize but I'll rarely ever get anything for my eggs, milk, etc. Occasionally, I do actually get the payout but I have no idea what causes it to work sometimes and what causes it to break.
Some people have suggested the mayor is too distracted by the stereo to review all the entries but it doesn't seem to matter whether she's judged my entry or not. I've seen her judge my minor entries plenty of times and still didn't get a ribbon, so I'm not sure the action of her judging the entries is actually linked to the results. It may just be one of those idle NPC animations to make the fair feel more realistic.
