Why won't my sim get pregnant?

That is a good question - I can only assume that since there is fertility treatment that they can possibly be infertile, but it could be a bug??

Try waiting a few days and try again. I have not had that problem, actually quite the opposite, I was just messing around with a couple gals I was friends with and chose Try for Baby just for kicks, now I have a few kids running around that are not part of my household!