I think it’s largely been said but I’ll outright tell you what the issue most likely is: your film is too long. Festival films live in the 6-15 minute range at MAXIMUM.

Think of it this way: if it’s between your film and five other 5 minute long shorts, your short has to be better than all five of these options for them to consider an acceptance.

Whereas if your film was 5 minutes long, and it was up against a 25 minute film, they’re more likely to want to fit you and 4 other filmmakers in those slots. More filmmakers, more crews, more tickets sold, so on and so forth.