A number of gaps were seen in the qualitative research on teachers using the TPCK model in our country, and this led us to conduct research focusing solely on Koehler and Mishra's TPCK model (2005). To this end, the purpose of this study is to identify the difficulties high-school teachers face when integrating technology into their classes, and understand the reasons for these difficulties within the framework of TPCK. The method used was the case study. The participants were five mathematics teachers. The data collection tools were lecture plans, class monitoring, semi-structured interviews and a focus group interview. The data analysis techniques used were document analysis, descriptive analysis, content analysis and speech analysis. The findings showed that the teachers have difficulties with components of TPCK (PB-PAB-TPAB) and that the difficulties they face in integrating technology into the teaching process can be sorted under five main headings: (1) "Teaching without planning"; (2) "Struggles to integrate different types of knowledge"; (3) "Lack of basic knowledge"; (4) "Errors in technological knowledge"; and (5) "Lack of field-specific support." In order for the difficulties teachers face to be minimized, in-service training focusing on the use of technology needs to become more integrated, provide teachers with knowledge applicable to the classroom, and be given by experts in the field.