The record
In a post dated June 4, 2016, a user says saying Hey Siri near their iPad Air 2 produced no response even with the feature turned on. The poster lists the software as iOS 8.1.2. They report running the setup several times, and later said they had tried the thread's suggestions, including changing how they said the phrase, without success. The poster added that setup finished and confirmed itself each time. The thread was closed without a confirmed solution. This is one user's report and does not show how often the issue occurred.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.