A high failure rate of changes during Post Implementation Review (PIR) in ITIL's change management process suggests a deficiency in the assessment and evaluation of change requests (A). Proper assessment involves analyzing risks, impacts, and feasibility before approving changes. If this step is inadequate (e.g., overlooking conflicts or underestimating impacts), changes are more likely to fail, as they may not align with objectives or be poorly planned.

Insufficient resources (B): May cause delays but is less directly tied to failed objectives compared to poor assessment.

CAB meetings not taking place (C): The CAB reviews changes, but the scenario doesn't indicate meetings are absent; poor assessment can occur even with CAB involvement.

Insufficient budget (D): May limit implementation but is less likely the primary cause of failed objectives.