I have a related field that is supposed to populate when I select a value from a drop down list, but this field isn't populating until I save my Incident.
Most likely you are missing a handler on the drop down field.
Open your window up in Window Manager and highlight the relevant drop down field.
Go to Properties and select to see the "Handlers".
If you cannot see this option, please see here in how to activate this.
In your "Available Handlers" list in the window that you get up expand "RelatedFieldHandler".
Select "UpdateRelatedFields" and move this to the "Configured Handlers section using the '[>]' button.
You will be presented with a pop up window where you need to select the relevant Event.
Make sure you select "ValueChanged" and click OK.
Click OK on your Handlers window and then save your Window.
Next time you select the relevant drop down list when logging an Incident the related field should populated on the fly.