Why don't you apply to fast food restaurants like Subway, McDonald's, Burger King, Carl's Jr? Those retail stores give you 20 hours a week, some of them even give you 30 hours a week, so you can earn a lot by working in there. You can quit the other two jobs that you have right now since they don't give that much hours. Or apply to a market as a cashier, they give at least 20 hours a week. Talk to the supervisors and the hiring managers when you apply to those jobs.