Table not updating with form
Open the form in Design view, select the combo, and then select the Data tab in the Properties dialog.
Make sure the Control Source value is set to the field in the underlying table where you want to store that data.
The link provided by Scott Mc Daniel assumes that your lookup tables have an ID and the lookup field (two columns).
Your statement is that you have tables with a single column (the lookup field).
Go back and insure the form/subform(s) are pointing to the correct table(s).