It's not clear if the query that runs OK is the source for the problem report. Assuming it is not, then if the report is based on a query, open that and verify that it is even returning the records as you expect. If not, that is where you need to start fixing things. If it does return all the correct records, then look at the report design. It may be a group by property is incorrect (grouping by last name, in which case there may or may not be records for 2 different first names) or the query is missing a PK field. If the PK field is LastName, that is bad design. If the PK field was an autonumber (included in the query) then each person would have a unique ID regardless of last name, and the report could then group by PK ID.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.