Complex sentence: Even though Rivero was featured in many Argentine films in the 1930s and 1940s, the early forties were a time of uncertainty for him, and bandleaders who did request him (such as Humberto Canaro) would not retain him for long.
Split into simpler sentences: Even though Rivero was featured in many Argentine films in the 1930s and 1940s, the early forties were a time of uncertainty for him. Even bandleaders who did hire him (such as Humberto Canaro) would not retain him for long.