First, take a look a this in case you want to change your approach http://allenbrowne.com/casu-23.html
Otherwise, make a query for the listobx that works if you write in each of the combo box values and test each. Once that's working, change the critera to reference the form combo (=Forms!frmFormName.ComboBox). Open the form, make a combo selection and run the query manually to see if you got that reference correct. If the query returns records for the combo selection as you expect, on the combo box After_Update event, make the listbox rowsource = that query and make the next statement requery the listbox.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.