VALENCIA, Spain — Over two years, Ana Maria Molina Cuevas, 36, has worked five shifts a week in a ceramics ­factory, hand-rolling paint onto tiles. But at the end of the month, she often went unpaid.

Still, she kept showing up. If she quit, she reasoned, she might never get her money. And besides, where was she going to find another job? Last month, she was down to about $130, with a mortgage payment due.