Production Scheduling

I'm implementing Production Scheduling and I have a question.

I do this steps.

1. Move a Work Order operation (e.g., drag-and-drop in the Gantt)
2. Mark it as Firm
3. Repair the schedule
4. Solve the schedule

But when I Solve PS move WO for other date…Why??? I put my WO in firm.

Thanks a lot for your help

María