1. The contract would not include the weakest in society, because they do not pose enough of a risk to be worth including. Or, one social group may be favoured over another. The unfavoured people could then be subjected to immoral treatment. This is not moral, so the social contract has failed to produce morality.
Examples: treatment of women
segregation in America
Germany under Hitler
Response: Even the weakest in society could potentially be a threat. Additionally, it is in our own interest to include them, because we may one day become severely disabled, or be part of a group which falls out of favour.
Response to the response: ...But, in the past, people perceived as 'weak' have been excluded. Women could not vote until the early 20th century; Hitler persecuted people because of their race. Even in the 1990s, Apartheid in South Africa led people to be persecuted on the basis of race.
2. The contract is only hypothetical, so there is no obligation to sign it. (Or, even if our ancestors did agree to such a contract, why should we be bound by it?)
Example: If a woman has a signed a contract with her employer, her children are not then bound by the same contract as her - they don't have to do her work for her.
Response: It is a tacit agreement that by being born into a country, you have automtaically signed that nation's contract. For example, if you are born in England, you are automatically obliged to follow England's laws, even though you never signed any kind of contract.
3. Dictatorship is not morality - dictators only rule in their own interests, not their people's…