Insufficient attention has been paid to behavioural influences on the implementation of the ‘Workload Control’ (WLC) concept – a Production Planning and Control (PPC) approach for small and medium sized Make-To-Order companies – and there is an implicit assumption that managers are rationalistic in their decision-making. This paper analyses the effects of both managerial decision-making behaviour and the size of a company’s order book, affecting the number of decisions that have to be made, on two case study implementations of a WLC system. The Recognition-Primed Decision (RPD) model from the Naturalistic Decision-Making literature is used to unpack the first case where implementation failed. This highlighted a misalignment between how the company’s owner-manager initially made operational decisions and how a rationalistic WLC system functions. But the company is studied over six years, allowing us to show how the owner-manager was forced to transition from the RPD model to a more rationalistic approach to PPC as the size of the order book increased. A second case study is then briefly presented in which WLC system implementation was successful; the RPD model was not strongly evident and the size of the order book was greater to begin with. The paper helps to understand the decision-making behaviour of managers in small companies and how it may conflict or be misaligned with the rationalistic assumptions underpinning the WLC concept. This provides a possible explanation for why few successful implementations of the concept have been presented in literature.