OK - I've poked around in your database a bit.
First thing I note is that there is no Row Source for "cboYear" combo box. So the filtering code that you're running on its AfterUpdate event wouldn't have a value to use, and thus the query will not return anything except if the "TMAP_OBD.vperiod_txt" field has an empty string (note that a Null will not be found by your code's query construction). In looking at the "TMAP_OBD.vperiod_txt" data, I see that every record has some type of string in this field, so the query will always return no records based on your current setup.
However, the setup that you're using is very limited. If your user decides to select the combo boxes out of order or decides to go back and change the setting of one of the "earlier" combo boxes, the code will ignore the values in the "later" combo boxes. I doubt that this is what you want.
I have a sample database that shows a reasonably easy way to create a "WHERE" string's contents based on whether values have been selected in a control or not. See http://www.cadellsoftware.org/SampleDBs.htm#FilterForm
for this sample database. It allows an unlimited number of controls/selections, and is based on "excluding" from the WHERE string any field where its "corresonding" control has been left empty.
I think this will be a better way for what you want to do.