The purpose of this study is to uncover and understand the factors that affect secondary science and mathematics teachers' initial implementation of Technology-Enhanced Formative Assessment (TEFA), a pedagogy developed for teaching with classroom response system (CRS) technology. We sought to identify the most common and strongest factors, and to understand the general process of how teachers adopt TEFA. We identified ten main hindering factors reported by teachers, and found that "time limitations" and "question development difficulties" are reported as the most problematic. In this paper we provide five vignettes of teachers' initial implementation experiences, illustrating different courses that TEFA adoption can follow. We classify our ten factors into four groups: contextual factors that directly hinder teachers' attempts to implement TEFA (extrinsic type I); circumstances that affect teachers' teaching in general (extrinsic type 0); gaps that teachers have in the knowledge and skills they need to adopt TEFA (intrinsic type I); and "ways of being a teacher" that describe teachers' deeper perspectives and beliefs, which may be consonant or dissonant with TEFA (intrinsic type II). Finally, we identify four general categories that describe the teachers' initial TEFA implementation.