Dawn,
Forgive me if I'm being to simplistic but your posting isn't clear to me; I can't understand why you need a combo-box in the subform.
In order for a subform's data set to track a main form's record there has to be some common field shared by both forms.
Starting with the main form lets assume you have populated a few fields among which is at least one unique field.
Now using the Subform Wizard (make sure you have the Wizards button on the Tool Box turned on) insert a subform onto the main form, selecting a TABLE as the source rather than an existing Form. Select the fields to populate the subform but note that at least one of the fields you populate the subform with MUST CONTAIN the same information as the unique field in the main form. This is the foreign key field in the Table that supplies the records for the subform. I usually make this invisible in the final design as its not often you'll want to see it.
The two fields do not need to share the same name - just the data. I.E. if you have a main table called PARENTS with a field called PARENT_ID then another table called KIDS will have to have a field which contains the childs Parent's ID in order to link the kids with the correct parents. The field in the KIDS table could be called SuckersID so long as it contains a copy of the related Parent's ID value.
As you progress with the Subform Wizard it will ask you which fields it should use to link the two forms. In fact it will probably suggest the fields to use.
Give the subform a meaningful name and click Finish. You should now have a subform that tracks exactly the contents of the main form. It may need a bit of tidying up but it should work.
It occurs to me that you may be using the combo-box on a subform to control which record is displayed on the main form. If that is so then I would suggest that you incorporate a drop-combo or list-box on the main form to select a record for display. Again use the Wizards to get a combo-box onto the main form that finds a record and makes it the current record. It's easier than programming it manually and works every time.
I hope this helps.
Regards
Rod