SAN DIEGO — In a typical last-minute scramble, Jannette Navarro, a 22-year-old Starbucks barista and single mother, scraped together a plan for surviving the month of July without setting off family or financial disaster.

In contrast to the joyless work she had done at a Dollar Tree store and a KFC franchise, the $9-an-hour Starbucks job gave Ms. Navarro, the daughter of a drug addict and an absentee father, the hope of forward motion. She had been hired because she showed up so many times, cheerful and persistent, asking for work, and she had a way of flicking away setbacks – such as a missed bus on her three-hour commute – with the phrase, ‘I’m over it.”

Jannette Navarro at Starbucks.

Newly off public assistance, she was just a few credits shy of an associate degree in business and talked of getting a master’s degree as some of her co-workers were. Her take-home pay rarely topped $400 to $500 every two weeks; since starting in November, she had set aside $900 toward a car – her next step toward stability and independence for herself and her 4-year-old son, Gavin.

But Ms. Navarro’s fluctuating hours, combined with her limited resources, had also turned their lives […]