Apple officially unveiled the Apple Watch to the public on April 10, but many are wondering of its limited supply. It turned out that the tech giant encountered some testing problems on the Chinese-supplied key component of the device, which is one of the reasons why it failed to prepare sufficient units.

People familiar with the matter told The Wall Street Journal that the taptic engine of the Apple Watch from one of the two suppliers was defective. As a result, Apple had to limit the production of the new device until the new supplies arrive.

The taptic engine is one of the key features of the watch. It is a combination of the "tap" and "haptic feedback," which sends vibrations to alert the user of the message. For instance, if someone sends you a text message, you will feel a vibration from the watch.

Luckily, Apple has two suppliers for the taptic engine. The defective supplies were from AAC Technologies Holdings Inc. of Shenzhen, China. The engineers discovered that the part collapsed during the testing stage.

Even if Apple is aware of the defective taptic engines, it doesn't have plans of announcing a recall because the company has already pulled out the completed watches containing the defective taptic engine prior to the product launch, the source added.

Apple is accepting online orders and is now experiencing challenges meeting the demands. For now, the company has its Japanese supplier Nidec Corp. to cover for the supplies, but it may take time before it can increase its production.

The company did not confirm the issue of defective parts, while AAC declined to comment. Nidec, on the other hand, was not available for comment.

 "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 Wall Street Journal. "We know many customers are still facing long lead times and we appreciate their patience."