Most issues are covered very well on government websites – see for instance https://www.gov.uk/maternity-pay-leave/overview.
We normally become involved where there are multiple issues. Typically problems can arise where an employer wishes to make redundant an employee who is on maternity leave or where the employer wishes to change the returning employee’s job description.
Failure to follow statutory rules or basic concepts of fairness can easily result in a discrimination claim.
It is worthwhile getting advice.