Zencart 1.3.9b www.ulster-scot.com

I added the above add on to the site some months ago and as far as I was concerned it was working fine. Recently we are having customers report that when they try to set up a new account the actual box asking for them to click on the downarrow and select from the menu where they heard about the site is not appearing at all. When they click to go to the next screen they get a message stating they havent stated where they heard asbout the site so cannot go to the next stage.

I have tried it on my PC with IE and Mozilla and it works fine and same with Windows XP on my laptop.

Why would this work perfectly for some people and not appear for others ?