The first place I would start is the query criteria to see what the query is looking for in order to return the desired data. If the criteria aren't being met for some reason then you wont get any results.
You mentioned that it didn't matter what the user's current group was, but if, for example, the query is basing its results on the user's primary group, and it's either blank or set to a group that doesn't receive assignments, then that could cause the query to not return anything.
It may also help to compare the user in question to other users in his groups, and see if he has the same privilieges as them.
Hope that helps.
Yes, these are all things that we started with. The queries work for the other members who are assigned to the same group and have the same roles. So I would conclude that the problem isn't with the group or the roles. What else can we possibly look at?