First you will want to make sure that your Before Update event is hooked up to run. By this I mean in your form the Before Update event should have an entry of "[Event Procedure]" (without quotes). You can have all the code you want in your db but without some way for the form to know to run it, it won't run.
Next, put a breakpoint in your code on the first line (click the grey margin on the left) then run the form. The code should break on the line you chose. At this point check the values of each combobox. I suspect there is a default value or null value such that your combobox isn't = -1 but something else. You can also "walk through" your code one line at a time with F8 to see how the code runs.