Perhaps they just reached their potential or their breaking point. Working on the same show for so long is tough. Most kids can't focus on anything for longer than 10 minutes. Most school projects are done at the last minute even when they had weeks to work on it.
Clearly there are good things going on in your program. Focus on those things and bring up the quality where you can. Find the areas where your group was weak and work on fundamentals to improve. Those small changes make a difference.
Don't let your disappointment show. They did a great job. Celebrate with them. Make it a big deal. They will want to do better next time but not if you give them a reason to feel bad about their results.