On courses like CELTA, TEFL or CertTESOL, there are some reasons why a candidate might not pass a productive skills lesson. Some of these reasons might be related to classroom management or planning.
The first reason is the absence of the productive skill stage itself! Imagine that! A speaking lesson without a speaking task or a writing task! This usually happens when candidates are carried away by all the previous stages in the lesson and their pace becomes very slow so by the time the productive stage is due, there is not enough time for students to write or speak. A good solution for this can be allocating 20 minutes for the productive stage in your planning and getting one of your colleagues to raise a sign for you when your teaching telling you that it is time get to the main aim immediately.
Another reason could be planning itself. Some teachers think that as long as their main aim is speaking or writing, all what they need to do is get students to speak or write. Well, of course this is important but if it is only what you are supposed to do, then you have got it all wrong. Because remember that students can speak to their friends in English using any social network application and they can also write emails at home as well. So why would they sign up for your classes if you will only provide them with what they can already do at home for free? In other words, your job as a teacher is preparing your students for the writing or speaking task. This means that you are expected to help your students with ideas, useful language, register and genre analysis so when they start writing, they use the newly-learnt elements in their production. And of course lessons where students write and speak without any real input from the teachers are lessons that hardly have any value for students.
To wrap up, passing a productive skills lesson means planning good student-support stages and leaving enough time for the writing or speaking task in the actual teaching.