Teacher involvement in curriculum design has a long tradition. However, although it fosters implementation of curriculum reforms, teachers encounter various problems while designing related to conditions set for the design process, and lack the knowledge and skills needed to enact collaborative design processes. Providing support to enhance teachers' design expertise is essential, since most teachers are novice designers. However, little is known about the nature of the support offered to improve teachers' design expertise. In this explorative study, six teachers and six facilitators offering support reflected on an enacted design process, the problems they experienced and the support offered. The findings indicate three gaps in teachers' design expertise related to three domains (1) curriculum design expertise, (2) pedagogical content knowledge and (3) curricular consistency expertise. The outcomes of this study illustrate the importance of supporting teacher designers during the design process and enhancing teachers' design expertise. By offering (tailored) support to teachers, the enacted design process and the quality of the design materials are expected to improve.