I wish I could help more, but see this thread.

http://forums.kingsnake.com/view.php?id=1874352,1874352

Also be sure to ask this in the sand boa and/or general boa forum. Be certain that your husbandry perameters are consistent, and that your adults are well kept and healthy. That's all I think you can really do. If this specific pair keeps having this problem, perhaps try a different pairing to find out if one of these parents has specific reproductive problems.

Good luck, I wish I had more advice for you.

Ian