What? That's like saying it's harder to walk to the shop than trek across the Sahara.
Well, listen to this, producer James Brown talked about the mountains of footage you inevitably get with a feature, lending you more to work with in case scenes aren't working in the edit. His film 'Still Alice' featured a 30-second shot of Julianne Moore waiting around, sitting on a sofa. This was shot before the sound had started recording, meaning Moore was actually not acting, she was literally just waiting for someone to shout 'action!'. It just so happened that this shot plugged a hole in the edit, and was in fact fundamental to making a scene work.
With a short film, every scene needs to justify its inclusion in the first place, so inevitably a lot of extra content just won't have been shot - there is less scope for thrifty reorganisation in the editing stage, as you have less to work with - on all levels. If what you've shot for a short film isn't working, you're most likely stuck with it, but if you're in the same situation with a feature, there's plenty of material to work with to fix it.
Lesson: if you're finding it hard to make a good short, that's okay - they're really hard, for everyone. Keep going, if you master shorts then you're in good stead for ultimately moving into features.