Not sure if it's applicable to your issue, but I've found that enabling the scheduling first (at the top right of the UI) before creating an actual schedule for the flow, causes the future job runs to not be scheduled. I avoid this by creating the schedule first, then turning the schedule on. You could try simply disabling and then re-enabling the schedule with the switch at the top right, in my experience that usually triggers Prefect Server to actually schedule the runs.