Like Utilitarianism, Contractarian Theories of Justice suffer from the flaw that they all require you, personally, to cut your foot off and sew it to your neck- or shoulder, if- like many ageing Social Choice theorists- you no longer have a neck.
Why? Well, there is some offer you would accept in return for undergoing this procedure. I make you that offer and throw in a 0.000001 pennies to every other member of your society. So kindly get busy sawing off your leg and sewing it to your shoulder.
What? You don't believe me? You don't think I'll make good on my promise to compensate you? But, if you don't believe me why should I believe any undertaking you give? Even if you have an unimpeachable reputation and have never cheated the million people you have dealt with so far- still, maybe you believed them and thus acted in a manner which caused them to continue to believe you. Me, I know you don't believe. How am I supposed to gain certainty that you won't betray my trust?
You may say- well that wasn't what I had in mind- the rules as I frame them forbid it. But, I reply, your rules as I frame them don't. Why should I accept you as an umpire when you won't accept me as an umpire?
You might then appeal to 'strains of commitment' and say well- Econ 101 and Psychology 101 and Biology 101 (all of which we're assumed to know behind the veil of ignorance) condemn what you suggest as an argument made in bad faith. The trouble here is that if Evolution is true, then it is the fitness landscape which decides what is or isn't an E.S.S of good faith/bad faith mixes. Think of Kavka's toxin- good faith may disguise its own bad faith because that's what made it adaptive- i.e. it wouldn't have evolved otherwise. Indeed, even assuming identical preferences, my forecast of a future fitness landscape may motivate my offer- you have no apodictic way of ruling it out a priori.
In any case, on the evidence of the last forty years, Justice as Fairness can't give us an answer to any problem we face in real life- but, happily, it can tell you- you personally- to get busy cutting off your foot and sewing it to your neck. (Your cheque is in the post- and always will be.)