Root cause analysis and CAR form

To answer your question, I will address your overall question of how to effectively implement corrective actions, which incorporates the step of root cause identification. 

To address corrective action effectively, you will start with a documented procedure which specifies which events are considered to be deviating (nonconforming), as well as the processes (sequence of steps) that the laboratory must take when complaints are received, nonconforming work is identified, and corrective actions are required. This procedure will apply to all laboratory activities that may be the subject of complaints or nonconformities and will, therefore, cover technical and non-technical deviations from the planned objectives of the laboratory. The assigned responsible person must decide, after initial investigation, if a containment (immediate) action is required; based on the impact or seriousness of the complaints or nonconformity. 

A Corrective Action form must be used to capture all incidents, complaints and nonconformities to ensure the correct personnel are informed and the root causes of problems are identified. Getting to the root cause could involve a number of techniques, where the laboratory will select suitable tools from those listed in the procedure, for the specific type of event. Either way, it is essential to clearly state the problem in order to get to the root cause and thereafter select the appropriate action that will correct the issue and prevent it from reoccurring.  

These articles will provide further guidance. Written for ISO 9001, they are also applicable to ISO 17025:

Seven Steps for Corrective and Preventive Actions to support Continual Improvement https://advisera.com/9001academy/blog/2013/10/27/seven-steps-corrective-preventive-actions-support-continual-improvement/
ISO 9001 – Difference between correction and corrective action https://advisera.com/9001academy/blog/2016/02/09/iso-9001-difference-between-correction-and-corrective-action/
How to use root cause analysis to support corrective actions in your QMS https://advisera.com/9001academy/blog/2016/03/01/how-to-use-root-cause-analysis-to-support-corrective-actions-in-your-qms/ 

These ISO 17025 templates could also be of interest: 

Complaint, Nonconformity and Corrective Action Procedure https://advisera.com/17025academy/documentation/complaint-nonconformity-and-corrective-action-procedure/

Corrective Action Report (CAR) https://advisera.com/17025academy/documentation/corrective-action-report-car/