The Wall Street Journal reports that the limited initial roll out of the Apple Watch was slowed by supply manufacturing issues concerning the Taptic Engine. The report claims that after mass production of the device began in February, reliability testing showed that some of the Taptic Engines supplied by AAC Technologies Holdings began to break down after use.

The Taptic Engine was created by Apple to produce the sensation of being tapped on the wrist as a less intrusive way of notifying the user. The engine uses a motor to move a small rod back and forth, which creates the feeling of tapping. The Taptic Engine also plays a role in sending your heartbeat to other Apple Watch users. KGI noted issues of the production of the taptic engine earlier this month.

Apple told some suppliers of other Watch parts to slow production until June, without giving an explanation, according to the report. Suppliers were surprised because Apple had recently told them that Watch inventory was insufficient, the report claimed.

The Apple Watch launch has been plagued by supply issues, with the device not being available in the company’s own retail stores at launch and shipping quotes being pushed to June almost immediately. During Apple’s earnings call earlier this week, CEO Tim Cook confirmed that the company hopes to launch the device in a second-wave of countries by late June.