If the problem is a manufacturing process, begin to establish a stable process. Once the process is stable, definition of the potential cause will be clarified.
If design causes are identified, screening experiments may help identify the key variables which are affected by subsequent processes. Design changes may be appropriate.
Four or five potential causes should be identified to investigate. Identifying several potential causes forces the team to address multiple possibilities rather than searching endlessly for a single cause. An implicit part of problem analysis is investigating potential causes in parallel rather that in series.