Hi,
I also had this problem before. After you enter new group and new interval, it still exist in system, but with another name differ from the name you've just enter. You can do it by this way:
1. Click on Overview button (Shift + F7)
2. Find out the interval which you've just enter
3. Remember the name of the system (your group name had been changed)
4. Maintain Groupd (F6) and you can rename that group by your own.
5. Assign Order type to that Group.
Good luck!