why couldn.t the clones get pregnant

The Hailsham kids are clones created by the government from the tissue of desirable ne’er-do-wells in order to provide organ transplants (“donations”) for non-clones with ailing livers and such. Although sex was encouraged, the engineering process made them unable to have kids.

As cloning is used in the "production" of livestock nowadays, we know that cloned animals can have natural off-spring. Even Dolly "had a little lamb" - more than one, actually. So why does the author decide his clones could not?

Consider the opposite: if the clones had children, that would be an extremely strong incentive for them to refuse donating and "completing". As it is, they only have themselves as a "family" and no relation in the real world. Especially as they are cloned from people who wouldn't go looking for them.

This last fact probably answers the question why they are clones at all. The donation programme could work equally well with normal children taken away at birth. Obviously, genetic resemblance to the receivers of their gifts is not the reason: nobody needs 4 donations, and moreover, the donors' "possibles" would then be respected members of society.

Coming back to my introductory sentence: what moral difference is there between producing organs to keep certain people alive and producing steaks to keep certain people fed?