Complex sentence: By 1970, both Parton and Wagoner had grown frustrated by her lack of solo chart success, and Porter had her record Jimmie Rodgers' "Mule Skinner Blues", a gimmick that worked.
Split into simpler sentences: By 1970, both Parton and Wagoner had grown frustrated by her lack of solo chart success. Wagoner persuaded Parton to record Jimmie Rodgers' "Mule Skinner Blues", a gimmick that worked.