Apple has said its new iPhone 5 sold faster initially than any other iPhone but sales have fallen short of some analysts’ expectations due in part to supply shortages, The Wall Street Journal reports.

The company, based in Taiwan with factories across China, uses the trade name Foxconn Technology Group, and assembles many Apple products including the new iPhone believes “the iPhone 5 is the most difficult device that Foxconn has ever assembled.”

“To make it light and thin, the design is very complicated,” said an official at the company who declined to be named. “It takes time to learn how to make this new device. Practice makes perfect. Our productivity has been improving day by day.”

According to The Journal, the executive said Hon Hai has taken steps to improve its productivity and address scratches on the metal casings of some new iPhones that were shipped. Hon Hai has recently implemented a new quality check procedure to reduce the chance of damages. But he noted the iPhone 5 uses a new coating material that makes it more susceptible to scratching.

“It’s always hard to satisfy both aesthetic needs and practical needs,” said the executive.

The recent spate of labor strife at the Taiwanese contract maker has raised concerns over whether Apple will be able to keep up with strong demand for the new smartphone. Hon Hai said earlier this month that two disputes between employees occurred at its manufacturing plant in Zhengzhou in China’s central Henan province, with neither incident leading to work stoppages. Hon Hai said it has addressed the disputes, which took place earlier this month amid a weeklong holiday commemorating China’s National Day. The company said the incidents involved a small group of production-line workers and quality-check personnel.

Hon Hai currently assembles the iPhone 5 for Apple at its plants in Zhengzhou, central China, and in Guan Lan, in southern China.

He admitted that the conflict between assembly line workers and quality inspectors at its Zhengzhou site last month was partly due to the metal casing and other “quality issues,” but he didn’t elaborate.

“The Zhengzhou site, which was set up in 2011, is still pretty new to us. We are still learning how to manage the work force there,” said the executive.

Since the Zhenghou site is a major production facility for the iPhone 5, Hon Hai has continued to increase hiring in Zhengzhou to boost its production capacity for Apple products and other customers, said the executive.