Forced to work less than my contracted hours

My daughter has worked for her employer in England for 17 months. When she was hired it was on a 25 hour a week contract working 9-3 pm whereas every other staff member (aside from the manager who also worked 9-3) was on 12 hour shifts. Other staff members complained about my daughter’s hours so they switched her to 8-2pm. They have never given her 25 hours a week. Some months she would only work 12 hours a week which left her way below her contracted hours. When she would ask for her full hours she would be told there was no hours available to give her but then they would be advertising for staff every week. She went on maternity leave on July 13th this year and took 2 weeks annual leave immediately prior to that. Since this April she’s had 62 hours annual leave. Or 2.5 weeks. Her contract states she entitled to 5.6 weeks a year but her boss is saying she’s had all her entitlement for the year and so they haven’t paid her for it.

If she had no entitlement why would they agree to give the leave in the first place? Her contract says she’s entitled to 5.6 weeks from March to April.

What are her legal rights in regards to being forced to work fewer hours than she’s contracted to and what rights does she have regarding her annual leave basically being refused AFTER they’ve allowed her to take it?

Her contract does not say anything about working less hours.

Thank you.