iMessage won't work on my WiFi

I visit my parents house where there is no mobile telephone signal. We have a good wireless internet connection. My iMessages to my friends usually won't send from here but I can't understand why. It can't be a problem with the phone settings because I can send & receive iMessages from the same people as soon as I am in rage of 3G. It can't be a problem with the internet connection because I can chat to them on Facebook instead & the connection is good as I can play games or watch TV on my phone.

Why will iMessages usually not send over the wifi here & how can I fix it? Any help would be appreciated!