Pulp mills often experience various forms of process impairment. However, the root cause(s) are often not obvious, and the mill personnel usually do not have the time and means to implement deep data analyses. Therefore, in many cases symptoms are treated instead of addressing the underlying causes, leading to recurring and expensive equipment downtime. Unscheduled downtime has been listed as a main threat to maximizing revenue in pulp and paper mills. We offer to implement root cause analyses on tenacious cases of process impairment in pulp mills based on a combination of expert knowledge, as well as machine learning and multivariate data analyses.