(I call this a 60 Second Screenwriting tip as it should only take about sixty seconds to read!  I know genius!)

What is a successful second act?  One that keeps the reader engaged, moves the story forward, and successfully delivers it into the falling action; that being the third act climax and the denouement.  A bad screenplay has a second act that simply doesn't keep the narrative trajectory in place and thus the spine of the story sags; meaning rising tension and conflict is not taking place.

The second act begins with a plot point that sends the story in a new and unexpected direction creating a central conflict that must be resolved.  The second act is the process of putting the protagonist through hell by placing obstacles in his/her way.  The second act mid-point is when all seems lost. 

The second act ends on a second turning point that brings a resolution and a climax to the central tension of the story. The second act story beats are the vertebrae of the narrative spine that rise and create drama and action that brings us to the second turning point.

Within the struggle faced by the protagonist in this second act you will often have a subplot that joins with the central tension and creates an epiphany whereby the character changes or grows.  By making this change the protagonist is able to overcome the central tension, the main obstacle, and defeat the antagonist.  

Hope this helps.  If so take my class, starts in a couple weeks!