If an occurrence is locked, it might be related to the user rights. Read more: Items - User Rights
Sub-occurrences (the sub-item of an item at the occurrence) might also show as locked since the relationship between the item and the sub-items is defined on the item and can not change. The exception is for when the sub-item is defined as a property Read more: Sub Items as properties
A sub-occurrence related to a child item can be swapped to define a unique sub-occurrence condition. See more: Swap Parent and Child Item