The middle. The second act can drag with subplots and episodic events. You need to give the middle of the second act, also the middle of the story, a bang. Ratchet up the tension/confrontation to get the audience involved in the story.
Here is where the protagonist has a pivotal moment. The lock gave the protagonist no choice but to go forward. Now, at the middle, she must change direction.
Key the middle to the end. If your hero will win at the end, this is the first time he finds a solution that seems to work. If the ending is tragic, then this is a very low point for this character.
Just as the middle mirrors the end, it contrasts with the main culmination at the end of Act 2, the event that leads toward the end.
At this point the protagonist has failed in her first attempts. Now he realizes the importance of what he is attempting and will face new trials and frustrations. After the midpoint you as a writer can turn up the frustration level. The hero may realize the right course of action to achieve his goal, but finds obstacles at every turn.
Just this much information should give you a key to the importance of outlining your story before you begin writing scenes. In the outline phase, you can make certain that obstacles increase in difficulty and challenge culminating in the final confrontation.