Second Combobox refresh

Hi I've been playing with an excel sheet that will help me put electrical quotes together using a database of items and costs.
The only thing I can't get my head around is how to activate the second combobox once the first combobox has been selected.

So for example:

If the first combobox is "Hager - Design 10" then the second box should display "Enclosure 4 Module DIN Rail White Steel".
My problem is the second combobox does not automatically update, only when I click in the box. Does anyone know how to fix this?