Why does my phone not sync or charge fron usb?

Hi all, i recently had to change the charging dock on my old iphone 3g. Since then i have found that although it charges fine from the wall socket it won't charge from usb and itunes doesn't recognise it.

I plugged my iphone 4 into the same usb and it gets found straightaway?

I have tried a soft reset and tried wiping it all to factory settings from the phones menu, but this has had no effect and now it is stuck on the screen that tells you to plug it into itunes. Any ideas?