there could be a number or reasons, because youve never had a loan the banks dont know how reliable you would be on repaying the loan over the agreed terms. it sucks but you would be more likely to get a car loan if you had a previous loan or were repaying a loan now - its like a track record.
best idea - leave it for 6 months (too many footprints on your credit check will go against you) before re-applying, request from the likes of experian (free for a few months online) your full credit report, this will show you if there are any anomolies on your report - eg your partner who has their mail sent to your house could be blacklisted - although it shouldnt count, they can sometimes use anything to refuse loans) as soon as you can, if there is anything wrong with your report, write away to them and get it updated.
then I would go for a credit card - capital one on say £250 limit, this will mean you can use it once a month (for petrol or a small shop) and pay it straight off again, this is building your score. If you owned your house, rather than rented, that would make the difference, you at present have nothing as insurance if you dont pay....
i never had any loans or cards and nearly had to pay in blood to get a credit score going, started with a cap 1 card, now Im on Platinum RBS with a £10k limit. (which I dont use lol)