I imagine you have put your voluntary work on your CV? I would possibly try to do some more to add to it. After doing a few months in a charity shop I managed to get a retail job quite quickly - possibly just coincidence, but I do think it helped me in my interview. Also, make a hobbies/interests section and add only those which will help your application - no silly random information. Things like social activities, group sports or clubs are good to include if you have any.
It could be an idea to get someone to do some practise interviews with you. Maybe you're just not answering the questions well enough? Getting nervous and just answering quite simply rather than expanding on your answers?