The great nature v nurture debate will run on for ever but in terms of what makes one person succeed and another not, then it's a very complicated thing isn't it?
I mean you need to have some idea of what success is. No easy thing in itself as one person's idea of it can be very different from another's. You only have to read some of the replies to Q's on success here to verify that.
Is the proof of a successful social mobility policy, the number of people from deprived backgrounds in 'top' jobs or should it be that people from those backgrounds have achieved above an expected average? We can't all be in top jobs, can we? And many people who are able enough to achieve a top position, chose not to for their own reasons.
It's difficult for people from challenging backgrounds to fit in to other social classes too. I've always considered my self to be working class even though most people wouldn't categorise me as such. It's a funny thing when you don't seem to fit into any class; it's a kind of hinterland.