As I replied to your email,
this quiz works: http://www.spanish-aid.co.uk/2011/06/05/quiz-of-the-day-ser-estar/ and this one doesn't: http://www.spanish-aid.co.uk/2011/06/15/quiz-of-the-day-practica-de-video/ So I think the problem is likely with your video plugin.
Also, there are a ton of errors on your site. You should start by disabling ALL plugins. Yes, ALL! Then enable mine. If mine works now, then there is a problem with your plugins. Reenable them one at a time until my plugin stops working, and then you will know the problem plugin. If after disabling ALL plugins mine still does not work, change your theme to the Twenty Ten or Twenty Eleven default theme. At this point, my plugin will almost certainly work, and you will know the problem is with your theme.
As many errors as you have, it is likely a lot of things. But if you turn on only my plugin with the default WordPress theme, it will probably work just fine. In which case, it's not really something I can fix because I didn't break it.