iPhone/iPad wifi sync backup not working automatically

Hi All

I have set my iPhone 4 and iPad 1 to synchronise with my local iTunes over wifi.

The issue that I have is that whilst the sync happens fine every time I plug my devices into a wall power socket (so apps, music etc all sync fine)... the actual backup process does not seem to be working. I noticed at the weekend that my devices do not seem to have been backed up for a couple of weeks.

When I right click on the device in iTunes and select back up manually, the devices seem to them back up to my computer without any problem.

I cannot understand though why they are not performing the back up over wifi automatically when plugged into a wall socket.

Is this behaviour standard or do I have a wrong setting somewhere??

Cheers

Gavin