If all of the companies that make activity-trackers were on some sort of leaderboard right now, who would be winning the race? None of them. The news yesterday that Nike had laid off members of its FuelBand division, and may not release future versions of the device, underscores the challenges players in this space have faced — and will face. Companies large and small are grappling with ways to cram a variety of sensors, batteries and other components into a device that offers enough value to not be tossed into a drawer after a month. Jawbone’s first UP wristband malfunctioned, resulting in a company apology and a no-questions-asked return policy. Fitbit recalled its rash-inducing Force wristband in February. Lark, which started out as a maker of activity-and-sleep-tracking wristbands, just introduced a personalized coaching app that runs on Samsung’s new Galaxy S5 smartphone. But Lark no longer makes Larklife, its clunker of an activity-tracking band, which launched in December 2012.