The iPhone 6S and 6S Plus, which boasted record weekend sales when they launched in September, are now facing weak demand, according to analysts, because they have fewer distinguishing features than their popular predecessors. The new iPhones also face tough comparisons with the strong sales of their older siblings. Apple tapped into a crucial market when it unveiled its bigger-screen 6 and 6 Plus phones in 2014, grabbing the attention of Asian customers, who had previously lapped up phablets from players such as Samsung Electronics .