The Wall Street Journal reports that Foxconn, Apple’s main manufacturing partner, may be struggling to keep up with the demand for the new iPhone 6 models. The company has reportedly been “churning out 140,000 iPhone 6 Plus and 400,000 iPhone 6” units every day, which is its highest daily output every, but the volume still hasn’t been enough for the number of preorders. Foxconn has apparently had to hire additional workers at its largest production size in Zhengzhou, and is continuing to do so. The site currently employs more than 200,000 workers and is dedicated solely to making new iPhones and related components, operating about 100 production lines around the clock. As in previous years, production is expected to gradually improve over time as manufacturers streamline their processes.