If my scripts were lives, there'd be terrible teens involved. And I'm not alone - many, many produced films I watch have the same problem. Everything falls apart about forty minutes in after a great start, and then the writer pulls it all together again by the end.
So what's to be done about second act woes? Here's the sentence that's been helping me: start early to ensure that your child grows properly. If there are problems in Act 2, then the origins of those issues are right there in Act 1.
Maybe your main character isn't set up right. Do we get a sense right away of who they are? Maybe the catalyst to send us into the next act isn't strong enough. Or the tone of the script isn't established from the start.
Whatever the issues, trace them back. Because the best way to fix your second act is to nail your first act.. The rest will follow...