I had to assign multiple no. ranges to multiple material type. Following action I had performed:
1) I expand all the no. ranges from group 1 where all the material types are attached.
2) I delete the no. ranges assign to group 1. By which group 1 also deleted (normal action)
3) I insert (F6) new group with one internal no. range for one material type and assign material type to that group.
4) I insert second group also system allows me but when I am inserting third group system neither give any message nor show me the third group.
Please can anyone know where I am mistaking.