Apple Inc.’s efforts to line up a second supplier for its high-end smartphone screens—and reduce its dependence on Samsung Electronics Co.—have hit a hurdle because LG Display Co. is struggling to make them, according to people familiar with the matter.
South Korea’s LG Display is working to boost production of organic light-emitting diode displays in hopes that it can provide some of the screens for iPhones set to be released later this year, the people said. OLED screens tend to be thinner and more flexible than those typically...