My sims in Sims2 aren't woo hooing right.?

They are married, adopted one baby, just turned into a toddler today, and they had a baby of their own that turns into a toddler tomorrow (sim day). The husband wants another baby but when I make them try for baby they dont woo hoo w/ the sparks or anything. They just move around under the blankets for a sec. and thats it. But she wont get pregnant. Why is it doing this?