The iPhone X will sport a feature that allows it to unlock when a user looks at the phone, though it appears it's this compelling new function that has limited Apple's ability to build enough units. It may also explain why Apple will begin taking orders for the phone next month before launching it in November instead of releasing it earlier this month with the iPhone 8 series devices.
The Journal said assembly of the two modules is creating an "imbalance in supply" and is a "bottleneck."
The report echoes a similar one from earlier this week, in which KGI analyst Ming-Chi Kuo said that Apple's iPhone X is facing production issues related to facial recognition technology.
CNBC reached out to Apple for comment but a spokesperson was not immediately available.