Some novice writers become so wrapped up in interesting events and bits of action that they forget to have a central unifying goal that gives purpose to all the other events that take place. This creates a plot without a core.
But determining your story's goal can be difficult, especially if your story is character oriented, and not really about a Grand Quest.
For example, in the movie "Four Weddings and a Funeral," all the characters are struggling with their relationships and not working toward an apparent common purpose. There is a goal, however, and it is to find happiness in a relationship.
This type of goal is called a "Collective Goal" since it is not about trying to achieve the same thing, but the same kind of thing.
So don't try to force some external, singular purpose on your story if it isn't appropriate. But do find the common purpose in which all your characters share a critical interest.
Referring to your story synopsis, have you included a Goal in which all the characters are involved?
If so, describe it below in as much detail as readily comes to mind.
If not, consider your subject matter and the activities in which your characters engage. What singular achievement would affect them all for better or worse?