Based on the SQL statement above, I'm not seeing any reason why your combo box would be limited to only one last name. I figured there would be an underlying DISTINCT in the SELECT statement - so my theory is blown out of the water.
Try setting the RowSource of the combo box to something like this:
SELECT [tblPatient].[SSN], ([tblPatient].[LastName] & ", " & [tblPatient].[FirstName]) As [Patient_Name] FROM tblPatient ORDER BY [tblPatient].[LastName]
Set the combo box properties as:
ColumnCount=2
ColumnWidth=1";1"
BoundColumn=1
By adding a command button to your form or using the combo box's OnChange Event you can change the forms RecordSource to:
SELECT tblPatient.Salutation, tblPatient.FirstName, tblPatient.LastName, tblPatient.Address1, tblPatient.Address2, tblPatient.Address3, tblPatient.City, tblPatient.State, tblPatient.Zip, tblPatient.HomePhone, tblPatient.WorkPhone, tblPatient.CellPhone, tblPatient.OtherPhone, tblPatient.NoMail, tblPatient.DOB, IIf(DatePart("m",[tblPatient]![DOB])=DatePart("m",Date()) And DatePart("d",[tblPatient]![DOB])=DatePart("d",Date()),CInt((DateDiff('d',[tblPatient]![DOB],Date())/365.25)),CInt((DateDiff('d',[tblPatient]![DOB],Date())/365.25)-0.5)) AS Age, tblPatient.Sex, tblPatient.[DL#], tblPatient.SSN, tblPatient.Phone, tblPatient.GeneralRemarks, tblPatient.AudiologistInititals, tblPatient.HAPatient, tblPatient.ReferalSource, tblPatient.Insurance, tblPatient.AltAddr1, tblPatient.AltAddr2, tblPatient.AtlCity, tblPatient.AltSt, tblPatient.AltZip, tblPatient.BillingAddress, tblPatient.BillingCity, tblPatient.BillingSt, tblPatient.BillingZip, tblPatient.Active
FROM tblPatient
ORDER BY tblPatient.LastName
WHERE tblPatient.SSN= & ComboBoxName.Value
(ComboBoxName.Value may require quotes)