Ina -
Nine times out of ten I find that I have a typo if things are not running properly. Quote in the wrong place or parentheses not correct, etc.  It's really hard to help you trouble shoot what is happening when we cant see what you have.
please copy and post your SQL here and that will help us move forward.

Also, open the tables and verify for yourself that you do indeed have data that should be showing up.  If there is truly no matching data that would be a whole different issue.

Are you receiving any error messages?

Hi Scott, when the form opens all the items are listed in the List Box. When I select a specific category, the list box is empty with no error message.

CategoryCombo SQL
Private Sub LogIt(Description As String, Notes As String)

    DoCmd.SetWarnings False
    DoCmd.RunSQL "INSERT INTO LogT (Description, Notes) " & _
        "SELECT '" & Description & "', '" & Notes & "'"
    DoCmd.SetWarnings True
    
    
End Sub

Private Sub Form_Open(Cancel As Integer)

    LogIt "Price Update Form Opened", "Blah"
    RefreshProductList
    
End Sub

Private Sub RefreshProductList()

      If IsNull(CategoryCombo) Then
    
        WhereStr = ""
    Else
        WhereStr = "WHERE Category = " & CategoryCombo & " "
    End If

ProductList.RowSource = "SELECT ProductIDID, ProductCode, ProductName, " & _
        "Category, LastUpdated, UnitPrice, NewPrice, Profit " & _
        "FROM ProductVendorNewPricingQ, " & _
        WhereStr & _
        "ORDER BY ProductName;"

End Sub

Private Sub CategoryC

The same SQL appears in the CategoryCombo and the VendorPriceUpdateF.  Is that correct?

In your RowSource:

ProductID is misspelled, you have an extra ID on the end. Also, you appear to have a comma after ProductVendorNewPricingQ.
Hope this helps.

Hi Kevin, ProductIDID was spelled like that in my table as well, although I've changed it now and I removed the comma and it still was the same. I then decided to add the SELECT statement in the CategoryCombo Row Source, which was empty, and it worked.  
Does it make sense?