Here is a situation: a boy meets an alien.
This is not a story because: 1. there is no evident conflict; 2. nothing much happens, there is no clear dramatic action. However, there is the third essential ingredient for a logline, a main character.
We turn a situation into a story by adding conflict and dramatic action.
A lonely boy befriends a stranded alien (dramatic action) and helps him escape govt. scientists (clear conflict) to return home (suggesting internal conflict since the lonely boy has gotten a friend).
See the difference? Write loglines about stories, not situations.