The reason Apple’s AirPods release date has been delayed has finally been revealed in a recent Wall Street Journal report.

Apple is apparently having trouble syncing audio between the two separate wireless earphones, a source familiar with the development of the AirPods told the Journal. The company is struggling with the wireless connection between the left and right pieces.

The AirPods are different from other headphone because they are completely wireless, with both left and right parts unattached. That makes the AirPods more complicated, since other wireless headphones transmit signal from one earpiece to the other. This is a problem because the two Airpods pieces have to stream audio simultaneously or else they will play uneven sound.

Apple is also working on what to do when a user loses one of the earphones, or if the battery dies in just one of the ear pieces.

AirPods Release Date

The AirPods will allegedly begin shipping over the “next few weeks,” it was recently revealed in an email obtained by MacRumors from a customer to Apple CEO Tim Cook.  

In the alleged email, the customer asks for the release date and Cook replies, "Sorry for the delay---we are finalizing them and I anticipate we will begin to ship over the next few weeks."

Apple has AirPods listed on its website for $159, but the earphones are currently marked as “coming soon," with no AirPods released date listed.

The AirPods were announced during Apple’s September event alongside the iPhone 7 and iPhone 7 Plus. The earphones were supposed to launch in late October, but the company said it still needed “a little more time before AirPods are ready for our customers."