Why do regulators cite an investigation that stopped at the failing batch?

Why

Section 211.192 says unexplained discrepancies and specification failures must be thoroughly investigated whether or not the batch was distributed, and that the investigation shall extend to other batches of the same drug product and to other drug products that may have been associated with the failure. It is one of the most cited parts of the drug CGMP regulations because investigations often answer "why did this batch fail?" and stop. The logic is simple.

Causes are rarely unique to one batch. If a mixer's seal wore and shed particles, every batch on that mixer since the seal began to wear is in scope, and so are other products made on it. If a raw material lot was contaminated, every batch using that lot is in scope.

The scope question has to be answered with evidence: which batches share the equipment, material lots, method, analyst or time window, and what data shows they are or are not affected. Inspectors read a narrow scope as a sign that the company protected the release schedule instead of patients. Q10's statement that CAPA considers other products that might be impacted, and EU GMP's expectation of root cause and CAPA effectiveness, point the same way.

The difficult part is bounding the scope. It should be wide enough to be safe and justified by the mechanism, not every batch ever made.