Zip Code custom function not refreshing

My Zip Code custom function (enter a Zip and the rest of address is filled in) is working, but the results aren't instantaneous. I have to click Refresh (F5 key on keyboard) in order for the City, State and Country data to change on the screen. Any ideas for fixing this? I'm trying to save my users time....it will defeat the purpose if they have to remember to refresh every record.

Thanks!