Combobox Results Incomplete

I have cascading comboboxes on my search form. The first selects a year, the second all hospitals with patients who had visits in that year, and the third is supposed to be populated with all patients who had visits at that hospital in that year. When I click on the third combobox, the results are incomplete. Some of the patients fitting the criteria are listed but not all of them. On each run there may be a different number of patients. If I just run the query by itself, all of the patients are listed. The combobox rowsource is:

SELECT DISTINCTROW [PatientNbr] & " - " & [PatientName] & " - " & [ServiceYear], [PatientNbr]
FROM qryFindPatients
ORDER BY [PatientName], [ServiceYear];;

The query (qryFindPatients) is:
SELECT tblSUR2941Master.ServiceYear, tblSUR2941Master.ProviderNbr, tblSUR2941Master.ProviderName, tblInterPatients.PatientName, tblSUR2941Master.PatientNbr
FROM (tblSUR2941Master INNER JOIN tblInterPatients ON tblSUR2941Master.PatientNbr = tblInterPatients.MedicaidNbr)
INNER JOIN tblInterPatientsEntries ON tblInterPatients.MedicaidNbr = tblInterPatientsEntries.MedicaidNbr
WHERE (((tblSUR2941Master.Sampled)="1"))
GROUP BY tblSUR2941Master.ServiceYear, tblSUR2941Master.ProviderNbr, tblSUR2941Master.ProviderName, tblInterPatients.PatientName, tblSUR2941Master.PatientNbr
HAVING (((tblSUR2941Master.ServiceYear)=[Forms]![frmIntermediateVisits]![txtRevYear]) AND ((tblSUR2941Master.ProviderName)=[Forms]![frmIntermediateVisits]![cboFacility]))
ORDER BY tblSUR2941Master.ServiceYear, tblSUR2941Master.ProviderName, tblInterPatients.PatientName;
How can I get all the patients to show up in the combobox?
Thanks in advance.