
Why do you say at 3 1/2 it will be too late to try again? In my breed we often would not even mate a bitch for the first time until 3 years (minimum 2), and advice is to have a first litter by five in case leaving it longer might predispose to whelping problems.
If you have used the same dog both times it might be wise to try a different stud, they just might not be compatible for some reason. Has he sired puppies recently, especially before and after your bitch?