Drop Down field looses it auto complete function when populated from a list.

Drop Down field looses it auto complete function when populated from a list.

I have a drop down field that i populate from a list. The list is created then sorted into alphabetical order and placed into the drop down field. Once I do this the list is correct, but the drop down field loosed its auto complete function. How can I maintain this feature as it is very useful to the end users.