Within minutes of online pre-orders opening for Apple’s the new wearable, some customers were informed that their Apple Watch won’t ship until June – a delay of five to six weeks over the advertised shipping date of April 24. A new report from The Wall Street Journal appears to explain why Apple experienced Apple Watch production shortages.Apple Watch production shortages.

According to anonymous sources with knowledge of the matter, taptic engines supplied by one of the two suppliers were found to be defective, resulting in Apple scaling back production of the Apple Watch.

Following the start of mass production in February, reliability testing showed that taptic engines produced by Chinese manufacturer AAC Technologies Holdings Inc. broke down over time. This resulted in Apple scrapping a batch of finished watches.

The report notes that Apple did not ship any of the defective watches; therefore, there should be no recall of Apple Watches.

Taptic engines produced by the second manufacturer, Japan’s Nidec Corp., passed reliability testing with no issues reported, noted the sources. Following the discovery, Apple has moved almost all of its production to Nidec. As it will take Nidec some time to ramp its output, this does not immediately solve Apple’s supply shortage.

The taptic engine is a unique part created by Apple for the Watch. A motor drives a small rod back and forth, creating a tapping sensation on the wearer’s wrist to alert a user of incoming notifications. The design is intended to be less obtrusive than a ring or buzz.

Pre-orders for Apple Watch opened on April 10 and Apple started shipping watches to customers on April 24. Apple is selling the Watch online and in a handful of designer boutiques, but not in its own retail stores.

Earlier this month Apple retail chief Angela Ahrendts issues a memo to retail store employees informing them that stores won’t receive stock of the Apple Watch until the end of May, citing “high global interest combined with our initial supply.”

The report from The Wall Street Journal also revealed that Apple is considering adding a second assembler for the Watch, Foxconn Technologies Group, to bolster output. Foxconn, who already assembles the iPhone, has reportedly started testing on Apple Watch production, but will not start manufacturing until later this year.

On Monday, Apple CEO Tim Cook noted that the company was, in many cases, able to deliver watches to customers who pre-ordered them earlier than anticipated. Cook also noted that Apple is on track to make the Watch available in more countries as early as June.

Screenshot: SiliconANGLE via apple.com