Almost, all possibilities have been given already. Let me have some summarized look on that.
- Check in OIOA for the option Release Immediately selected
- Check in Maintenance Plan for the option Do Not Release Immediately not selected
- Do the scheduling in IP10 & save the scheduling
- Create an Order with same order type in IW31 with same task list & release the order manually
- Check the validity date of the Task list & Basic Order date
- I hope, no developments have been added to that order type or maintenance plan scheduling
Please check all the details & let us know.