@CarArena Hi Ashley! A few possibilities:

1. Make sure you're using the iPhone 4 cable that
came with the charging plug for the phone. Other 3rd Party cables aren't always compatible with all Apple iPod/iPhone generations.

2. Make sure that the car is running with the EPB applied and insert the Apple USB cable into the slot within the armrest. Insert the iPhone to the Apple cable, the iPhone may advise that the phone software is not compatible with the vehicle, but press "Dismiss" on the iPhone.

3. The USB select option on the infotainment unit is with the CD button, i.e. if listening to the radio and there is a CD input too, then press CD once for the CD and twice for the USB input, i.e. iPhone for iPod input.

I have a unit myself and another possiblity is that, inputting the
iPhone lead into the Phone 2-3 times a day can wear the contacts of the Apple USB cable. I have had to replace my once already after 12 months continuous use.

Hope that helps a little, if not tweet me back Ashley! Eseentially, if none of those work, it will have to be popped back into a Vauxhall dealership for an inspection. Thanks ^MA