Never, never, never trust the incubator's readings. For coturnix quail, 40-50% humidity is a bit high, but should still be okay. It's a good humidity for hatching time, so keep it there for now.
I would candle them again to see how they look. Since your chicks are late, I'm guessing that your temperature was low during incubation. They still have a chance, but if they haven't hatched by day 20, they're not likely to.