Workplaces are considered promising settings for protecting and promoting the health and wellbeing of employees. To date, few workplaces, particularly small and medium sized enterprises (SME), or their workers, have adopted Workplace Health Promotion (WHP), raising questions as to why. We conducted interviews in 10 SME in the Australian Capital Territory (ACT) asking managers and workers to reflect on the reasons for their participation (or not) in WHP activities. We qualitatively explored factors that managers consider important when deciding whether to offer WHP and compared these to worker considerations regarding participation. Both workers and managers identified time constraints as a major barrier for participation in WHP activities. If WHP was implemented, time constraints arose mainly from: difficulties scheduling and synchronising activities to include most staff, even then casual staff were mostly excluded; and the duration of time required by employees to participate in the offered activities, and whether this was in paid (work) or unpaid (worker) time. Workers struggled to participate in WHP in their scheduled breaks and were reluctant to use time outside of work hours. We have developed a model illustrating the emerging tension between managers' and workers' needs for WHP participation. WHP participation will likely remain low until this tension, and associated financial implications, are widely acknowledged and addressed. Our research indicates that time should be considered more explicitly and thoughtfully in the workplace engagement and WHP design, to actively respond to constraints from activity duration, scheduling and synchronisation.