Sims can't try for baby

I have this same issue and haven't been able to find a solution. I have read every thread about it I can possibly find online and none have solved it.

I have two Sims who started dating and then moved in together - one is custom created, one came with the city living expansion pack.

They have maxed out romance and friendship scores, they have a fully accessible double bed in the apartment and I've gotten them flirty etc. They CAN woohoo, but there is no 'try for a baby' option. Other sims in my game in other couples DO have the 'try for a baby' option, including ones with much lower relationship scores.

- they are both young adults

- I have checked on CAS that the male sim is capable of getting sims pregnant and the female sim is capable of getting pregnant

- I've tried the CAS edit cheat suggestion where you change their age - still doesn't give the baby option

- I have tried going to another lot to see if they can try for a baby in a bush or anything, but still only woohoo shows

- I have quit the game/changed household/etc and still have the same problem when I come back to them

- I don't have any mods or custom content, or anything downloaded beyond the base game and expansion packs

I'm completely stuck, no idea what to try next. Can anyone advise???