Hi Sarah,
Generally, if there is any issue with the drilldown, it's as a result of there is an issue with the locancestor, but you also need to check the lochierarchy. They are used in different situations.
LOCHIERARCHY is easy as that is just LOCATION + PARENT, whereas with LOCANCESTOR is LOCATION + LOCATION, LOCATION + IMMEDIATE PARENT, LOCATION + IMMEDIATE PARENT PARENTS, etc...
Though it's a little hard to understand your issue without a picture.
Is the problem in the standard drilldown or the advanced drilldown by the way?
Regards, Craig.