Perhaps you didn't give enough background or context. A story needs a solid foundation to expand. Maybe you also didn't explore the emotions and motivations of the characters thoroughly, making it seem too brief.
It could be that the source of the music you used had a limited duration available. Or, it might be that the software or platform you're using has some default settings that capped the music length at 5 seconds.
The shortness might be due to not spending enough time on descriptions, dialogues, or developing the conflict. You could also look at the pacing and see if it's moving too quickly.
It could be that you haven't explored all the possible subplots or character arcs. Also, the lack of descriptive language can make the story seem shorter than it could be.
It could be that your pacing is slow. Not enough action or progression in the plot might make it seem longer. Also, maybe you're spending a lot of time on character development or background descriptions.
It could be that you rush through the storytelling or don't spend enough time fleshing out characters and settings. Also, lack of diverse events or complications can make the story seem short.
Maybe you're not using all the available time or features. Try adding more content or using different formats to make them longer.
Perhaps it's a story that focuses on one simple action or emotion that can be expressed in just 3 seconds, like a sudden surprise or a brief moment of joy.
Maybe there's a limit set by the IG platform. Sometimes they have specific rules for the duration of stories.
Perhaps in those 15 seconds, you were sitting at your desk, staring at a difficult problem on your work or study. Then suddenly, an idea popped into your head, and you quickly jotted it down, feeling excited about the possible solution.
This theory originated from a legend that fish only had seven seconds of memory. The saying goes that when a fish is placed in a new tank, it may adapt to the environment and remember the environment within seven seconds, but it may forget the previous environment for a period of time. This saying has been quoted in some literary works, such as a famous line in William shakespeare's play hamlet: "time is the best medicine, but also the shortest medicine." This could also be explained by the fact that fish have short memories because they need some time to adapt to a new environment, unlike humans who can retain memories.