In my experience, how about checking the recurring frequency? It is more about seasonal issue or more depending on shift and/or personnel. Might be following the specific raw material's lot change. Or it could be only during cold start-up. With an open mind, at least starting run-chart for any possible root cause.

Recurring quality control failures in a chemical plant can significantly disrupt production and harm profitability. Addressing these issues requires a proactive, systematic approach: 1. Root Cause Analysis (RCA) 2. Standardized Operating Procedures (SOPs) 3. Enhanced Staff Training Programs 4. Real-Time Monitoring and Automation 5. Cross-Functional Collaboration 6. Supplier and Raw Material Quality 7. Preventative Maintenance 8. Metrics and Continuous Improvement 9. Management Commitment By addressing the issue at multiple levels—process, personnel, and technology—you can minimize quality control failures, reduce disruptions, and protect your bottom line.

This is fairly normal at some point in almost all manufacturing facilities I have worked in. There can be many reasons why we have quality control issues, sometimes it's supplied materials & products, in this case it's all about dealing with it at supplier level. Where it's a internal issue in a production process, then we must perform analysis on the non conformance, performing root cause analysis is the correct way, you may want to carry out a thorough inspection on raw material, and inspect samples, in set qty's and times, if it's in spec then it's in process production where quality is being lost. If this is a big issue then process map, and put controls into place. You may want to consider SPC to plot process and to control it.