Ok but before you go throwing random stuff into your story to spice it up or get it un-stuck, consider doing the following:
Grab a few events (minor or major doesn’t matter) from earlier or later in the story and trace out the causal chains. What caused it and what did it cause? Can the domino effect lead to a new event?
Trace out the “story” of the main cast’s motivation. How does the motivation interact with the story and how does the motivation change over time?
Go hunt for story elements you put in earlier that could be escalated into subplots.
Take a few characters who have different levels of information (or are more or less close to antagonists) and go through the story from their perspective. Perhaps you don’t know what should come next for your main character, but it might be obvious what comes next for a different character.
When you throw in new characters, first try to repurpose old characters. This makes them feel less cheap and gives them a better chance at developing depth. And when you do want to use a totally new one, consider someone who’s related to an existing character or a causal chain. The reader might already be wondering where X character’s parents are, after all.
Point is, the deeper the connections between your story elements, the more satisfying the read. You don’t have to view those connections as a constraint. They can tell you what needs to happen next.
