Examining the situation using cause-and-effect analysis, interviews, discussions, inspections, and observations will generally reveal the problem source.
Depending on the source, find an appropriate response. Performance problem resolution is necessary if the individuals doing the work are at fault. If the inputs to a process are faulty, resolution will require better monitoring and control of those dependencies. Other situations may require recalibration, replacement, or repair of equipment used in the development process. If the process itself is suspected to be the problem, a process audit as part of ongoing quality assurance or a process improvement project might be the answer.
Select the most promising approach and document it.