Is it normal to be good at writing novels and long stories but not short stories?

I would say so. Mark Twain has been quoted saying "I didn't have time to write a short letter, so I wrote a long one instead". Trying to write short stories isn't always easy because you have to capture the most important parts with as little words as possible. This is why many short stories can speak metaphorically, or are open to interpretation. As long as the main idea is there that's all that matters.