In this paper, we will focus on the effects of teachers’ lack of the algebraic knowledge and sensitiveness that are necessary to effectively control the plurality of levels of interpretation involved within activities aimed at making students aware of the meanings associated to their use of algebraic language. Our analysis of class discussions conducted by a group of middle-school teachers involved in university training courses enabled us to highlight profound gaps, between modeling and interpretation, that prevent them from becoming aware of what it is important to stress when facing this kind of activities with students.