RPX (NASDAQ: RPXC) set out to solve a huge problem: how to deter patent trolls from attacking large corporations through intellectual property litigation. Yet as valuable as that service has been, RPX has recently had trouble finding ways to grow its revenue from subscribing clients. Coming into Tuesday’s fourth-quarter earnings report, investors knew that RPX’s relatively new discovery services business would lead to an overall sales increase, but they still wanted to see signs of growth elsewhere.