The second act is the longest of the three acts and for most people, the hardest to write. It's easy (usually) to come up with the beginning of a story and the big climax, but what do you do in between? This is where many stories bog down.
One thing you can do is keep in mind that every scene in your movie must move the story forward and in the second act, you need to keep increasing the tension and conflict. You must continually increase the stakes until a "showdown" is inevitable.
Yet at the same time, right at the end of the second act, things need to be at their worst for your protagonist. Often this is done by having your hero experience their "dark night of despair," when everything seems lost and or they have lost faith in themselves.
This is how you build a second act. When you construct it this way, the contrast between the dark low point and the climax will make the ending much more satisfying for the audience.