Faulty parts from an Apple (AAPL) supplier are slowing down shipments of the company’s new Apple Watch, the Wall Street Journal reports, citing people familiar with the matter.

The problem lies in the Apple Watch’s taptic engine, a tiny component that allows Apple Watch notifications to simulate the effect of being tapped on the wrist. Taptic engines from one of two suppliers, the Shenzhen, China-based AAC Technologies Holdings, were found to quickly wear down “after mass production began in February,” the Journal reports. Rather than ship potentially bad units, Apple decided to throw away some Apple Watches with the AAC taptic engines.

Apple’s comment to the Journal didn’t directly confirm or deny the report. However, this is most likely Apple’s attempt to explain ongoing Apple Watch shipping delays to frustrated shoppers. While Apple initially said the Apple Watch would start shipping April 24, many buyers have reported estimated shipping dates as far back as June. However, some of those buyers have said their Apple Watch shipments are coming faster than Apple estimated.

“Our team is working to fill orders as quickly as possible based on available supply and the order in which they were received,” Apple told the Journal. “We know many customers are still facing long lead times and we appreciate their patience.”