If the students are saying that they already know the structures and you see that they’re not using them properly, you’re right, something needs to be done.
“Knowing a structure” is a complex thing. It involves knowing how to form the tense (or other language point), when to use it, when not to use it - amongst other things. It also covers being able to produce it easily in conversation. My guess is that the students know the form of the present continuous, comparatives, etc but they are not producing them correctly in speech. This is often the case at pre-intermediate level. When the focus of the activity is, say, the present simple they can get it right. When the focus of the lesson moves to something else, they can slip up and make mistakes.
It sometimes helps if you make this distinction clear to the students. Explain that you’re giving them more opportunities to practice the structure in class, not teaching it anew.
Also think about how you are reviewing language in class. They don’t need a full-blown PPP lesson: presentation by the teacher, controlled practice and free practice or production. Think of other ways to initiate the use of the language. You can use texts or listening material that include the target structure. The focus of the lesson is skills work with a chance to review the language point. Give the students opportunities to extract the language points and / or deduce the language rules for themselves. Set up speaking practice, role plays, discussions etc where the language is likely to occur naturally. They can also teach each other the rules or explain them to you. This way everybody is happy – the teacher sees that the language is being reviewed and the students get a chance to recycle the language in a new and different way.