A growing number of users reported that the iPhone X touchscreen takes up to 10 seconds to turn on when a call comes in, preventing use of the on-screen buttons to answer or decline a call. In online Apple forums, iPhone X users outlined the issue:

“I can often not see an incoming call coming, can only hear it! At other times, there is a significant delay :(“

“Whenever i receive an incoming call in my iphone x , ringtones starts but the display comes in after 6-8 seconds. Whenever i restart my iphone x the problems gets automatically solved and after 15-20 calls the same problem starts again”

Apple (AAPL) said it was looking into the reports.

iPhone X was the most popular phone for the holidays, shipping 29 million units according to the market research group Canalys. But sales of the device fell in January, leading to Apple’s first market correction—a 10% stock price decline in sequential days—in over a year.

This was not the first sign of trouble for the latest iPhone. Last year, production delays pushed back the launch of the phone and though the company’s latest earnings report shows a record quarterly profit, unit sales of the iPhone are down. At the end of January, the company reportedly planned to cut its production target for the device in half due to poor sales.

This story has been updated to reflect Apple’s comment to Fortune.