Worried producers allegedly blame The Voice’s failure to produce a big-time star for plunging ratings. According to a recent report, insiders insist the fault lies with the NBC show’s celebrity coaches, who were accused of not knowing how to pick talent, RadarOnline.com has learned. Since the singing competition show launched in April 2011, country king Morgan Wallen has proven to be the only former Voice contestant with a significant music career — and he was booted by ex-coach Adam Levine before the live performance round in 2014. In contrast, its rival show American Idol has generated super…