There is no action in the opening salvo. Perhaps you've become so interested in setting the scene, you've spent pages on the setting. Or there is a single character doing nothing but ruminating on what to do next or on the weather or something that shows no action.
After a brilliant opening paragraph, you've jumped immediately into a flashback that goes on for pages telling what happened before in your character's life. Now this is information you as the writer might need but not the reader, at least not in a huge chunk.
Every story starts with some kind of change but you may have started pages and pages before the change occurs. A change in a character's situation is what triggers the story.
You've opened with some action, little backstory but the reader has no idea of where in time or space the characters exist. Establishing this can take only a few words but if missing, the reader may have thought they were reading a mystery when it really was a romance,
The opening scene is a big bang but then the action falls short.
The mob scene opening when there are dozens of characters introduced but no one really knows who the main characters really are.