There will be something wrong with the "linking" you are using.
Bascically, a sub form is like opening a form with say a macro or code that runs OnCurrent. In other words it is like saying "please open the form for me where all the records in Field XYZ are the same as FieldXYZ on my MainForm"
If you were link with something like LastName then when you came to Smith or Brown in your MainForm then the subform would disppaly all of the records with Brown.
If you don't have some sort of ID Number field and have to use Name then you should in a query create a new field like this:
NewField:[FirstName] & " " & [LastName] or similar and then this can become your linking field.