The Apple Watch includes what the company calls a taptic engine, which simulates a tap on the wrist to get the wearer’s attention. The Wall Street Journal is reporting that the engines made by one of two suppliers—AAC Technologies Holdings Inc., of Shenzhen, China—have failed reliability tests and could break down over time.

The problem could slow the rollout of the Apple Watch as Apple shifts production to its second taptic-engine supplier, Nidec Corp. of Japan, the Journal reports, adding that Apple won’t be recalling any watches, because none with the potentially faulty engine have shipped.

Daisuke Wakabayashi and Lorraine Luk in the Journal point out that the taptic engine problem highlights a potential downside of Apple’s lean supply chain. “Apple can produce massive quantities of products with little waste and excess supply, but it can experience shortages when a problem arises with a key part,” they write.

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