Despite the existence of many studies about the different aspects of fixation in creativity and design reasoning, the underlying mechanisms of fixation, i.e., the processes that interfere during creative reasoning and lead one to become fixated on a small number of unvaried solutions, remain unclear. In this paper, we propose a theoretical framework to model fixation based on C-K design theory, which states that fixation is characterised as a set of restrictive heuristics activated in creative reasoning. We applied our framework in a set of experiments. We demonstrated how this framework makes sense of the varieties of fixation in design processes. We conclude by proposing three capabilities to understand fixation and overcome it: restrictive heuristics development, inhibitory control and expansion.