Modules & VBA :: DLookup Only Reading Top Row

I have a command button on a Access 2010 form that i am using as my switchboard. On this form i have a hidden unbound text box that captures the users environ"username". When the user hits a command button on the form the code looks at the name in the hidden textbox that captures the environ"username" and then DLooks up a table to see if there is a match. If yes then it will open the next form and if not then a message box appears.

Code:
Private Sub Command6_Click()
Dim TxtUsername As String
If Me.TxtUsername = DLookup("[OneLondon Login]", "TblAccessUsers") Then
DoCmd.OpenForm "Bakerloo_Main_Form"
Exit Sub
Else
MsgBox "You do not have permission to access this database"
End If
End Sub

This works fine apart from the fact it will only read the first name entered in the table. This table(TblAccessUsers) could have up to 50 names in it and possibly have names removed and re added at a later date. Is there a way i can get the code to look up every name in the table ??