In my opinion: I may be wrong, but here’s my take. Perhaps Brocklehurst (PhD) relied on confidence to execute the speech plan. If so, subconsciously his brain was constantly checking (i.e., evaluating) a confidence level to decide when to execute speech. For ten years that confidence level stayed above the subconscious threshold Brocklehurst (PhD) relied on, so his subconscious didn’t feel the need to over-regulate speech execution — there were no stuttering manifestations. In short: his confidence was high enough that the protection mechanism didn’t engage. Then, after moving to another country, the confidence level (and other STIMULI) dropped. Examples: a reduced confidence level to execute speech (lack of confidence to regulate speech execution, or reduced confidence to effectively fine-tune when the subconscious SHOULD execute the speech plan). So. There could be dozens of subtle varieties of confidence and related other STIMULI that most stutterers are not even aware of, yet that interact with their stutter/PROTECTION mechanism. So after the move to another country, the brain began relying again on A CONFIDENCE LEVEL to excessively regulate the execution of the speech plan — i.e., it started to excessively manage the release threshold mechanism. The book says that once stuttering returned, MINDFULNESS didn't have the effect it once had. In my opinion: I'd say this could imply that applying mindfulness was not effective to deal with the PERCEIVED (or evaluated) confidence level that the subconscious was relying on to execute speech and manage its speech execution regulation. There could be many reasons for that. For example, as Brocklehurst (PhD) himself stated in his book, that he seemed to lack the motivation (or effort, or intention) to bring stuttering back to full stuttering remission (as it once was). That exact narrative (or attitude, or cognitive distortion) could be one of many reasons MINDFULNESS stopped being effective for the specific stimulus (confidence) driving his protection mechanism.