Why is my OnSelectedIndexChanged not triggering?

I have two <select> controls being used as dropdowns: one with a collection of cities, and the other with a list of hotels. What I would like is that, when the user changes the selected city, the hotels dropdown updates to only show hotels in that city. The drowpdown with the cities is populating successfully; however, I can’t seem to get an event to fire when the city is changed.

ASP .NET Button event handlers do not fire on the first click, but on the second click after a PostBack

Background: I am customizing an existing ASP .NET / C# application. It has it’s own little “framework” and conventions for developers to follow when extending/customizing its functionality. I am currently extending some of it’s administrative functionality, to which the framework provides a contract to enforce implementation of the GetAdministrationInterface() method, which returns System.Web.UI.Control. This method is called during the Page_Load() method of the page hosting the GUI interface.