The culprit was the taptic engine, a component which Apple designed to simulate the sensation of being tapped on the wrist. Reliability testing revealed that some Taptic engines – supplied by AAC Technologies Holdings – began to break down over time, according to a Wall Street Journal report citing people familiar with the matter.

The second supplier of the component – Nidec Corp – did not seem to have the same problem, as the Taptic engines supplied by the company did not show the same symptoms. The sources told WSJ that Apple has moved almost all of the engine production to Nidec, but also said that it might take time for the company to increase production.