I hate to ask the obvious, but if they are never trying for baby, are you sure you've set it to allow try for baby on the bed? Isn't try for baby off by default on the bed?
It's on.
When I was *testing* the bed to see if it would work for my purpose, I did have sims Try For Baby (though no pregnancy resulted), but since then I've never caught two sims doing it. So I know it's not broken, they're just not choosing it. I'll have to check if my sims pass all the tests. Maybe that is the problem.
I'm thinking I'll try to solve my problem by removing the Woohoo option from the bed, leaving just the Try For Baby.