Not necessarily. In a restaurant, even if you ask for full time hours, you might either trade them away or find a need to consistently request days off to the point of the manager scheduling you way less.
Scheduling in restaurants is super complicated and comes down to a mix of performance (sales) and consistent availability. No one is ever scheduled for 40 hours either because of the potential of a shift going longer. Overtime kills budgets and forecasts. I was once cut in mid shift to avoid going over 40 hours.