If the parent of a WTPart are the non-latest version of the parent, the parent will not be displayed in the 'Where used' list, as this list uses latest.
Article CS21060 describes that parent can be returned by creating a change request and 'collect affected objects'. Article also describes the problem with 'Where used'.
We do not use the change-module, but really see the need to be able to detect parents for our WTParts, even if they are not included in latest version of parent. The scenario can even be that WTPart is included in a released part config, but in later version of parent, the WTPart are removed from BOM. As funcionality is now, it is not possible to find this parent
We are using 9.1 M070, but as I understand from support, the functionality is not included in 10.1 (yet)