If the dog was pregnant when she left the rescue and they didn't tell you that was a possibility then contact them and ask for help. (they can only say no which i would hope they wouldn't
)
Given the amount of Staffies in rescue including pups it's not suprising they aren't selling.
How much are you asking for them.It's such a difficult balance as clearly you don't just want to give them to any old Tom Dick or Moron that turns up at your door but every day that goes by people are less likely to want them and will look for a younger pup elsewhere.
What a difficult situation to be in. I hope you find them all good homes.