Hi,
Employer should give opportunity to new people in new shift, if existing employee is not interested to work. Employer can not force employee to do overtime. It's all about Manager to whom these people report. Manager must have skill to motivate employee to work extra time and provide opportunity to employee to earn extra. I would suggest you to do self introspection to find out why these people are not interested to work extra.
Are their salary is too low?
Are they not happy with their senior?
Why they are not happy with seniors?
Working atmosphere is good or not?
Are they overloaded with work?
Are they doing same work repeatedly because of other mistakes such supplier quality issues?
Does your company pay Overtime on double rate?
Does employer take care of employee?
There are many questions
If you will put the restriction on employee they will leave the job and then ask your employer do it by himself.