Why do my sims leave the bed when both are trying to sleep?

So this seems to happen a lot in one of my save files. I have 2 sims that are married. Well the father is a divorcee and so his twin boys are from his ex wife. When she had the twins there was no problem and they could sleep in the bed together with no problem, but he has recently gotten re married and his new wife has had 2 girls. Before she had the first girl they could sleep fine, but then she had the baby and they wouldn’t sleep in the same bed without waking up immediately and once the baby aged up and got her own room they were fine. Well she just had another baby and the same thing happened, but this time they still won’t sleep together. Even with the toddler in her own room.