iPhone X contains the Romeo and Juliet modules required for Face ID to work. Romeo is the codename of the dot projector, and Juliet is the IR cameras installed on iPhone X. According to The Wall Street Journal, Romeo has fewer modules than Juliet.
Romeo projects over 30,000 invisible dots onto your face, creating a unique map of your face. The IR camera then analyzes the dot pattern and performs a compliance check. Naturally, these components cannot do without each other, therefore the lack of one of them stops the entire production.
iPhone X should be available for pre-order on October 27 and arrive in stores and customers starting November 3. So far, Apple hasn't explained the delay compared to the iPhone 8 and 8 Plus, which went on sale last Friday, but production difficulties probably played a role here.
The reason for this is a lot of attention to iPhone X. Apple will obviously have to provide their best smartphone with significantly more buyers than the other two models.
This increased demand is driven by the many technological innovations inherent in iPhone X. Face ID is one of the most important of these, so it is hoped that the modules will be shipped as soon as possible.