My two sims, that have conceive a child, for some reason can't seem to be able to propose to one another. When I do a romantic action, I notice a broken heart thought bubble above the ex-mascot girl. I checked her relationship after having her ask her other lover to be just friends, she doesn't seem to have any other romantic interact other than the sim in her household that I'm trying to get them engaged. Halp?!

If you use woohoer you can enable debugging and it will show you why certain romantic options are greyed out. I use this when i run into issues like this bc ive been tweaking my settings.

Are your sims in a good flirty mood? I've noticed in my game that I can't propose unless they are already declared boyfriend/girlfriend and are finding each other "irresistible". So try and spice up their date ^.~

Had resolved it with nitromon's suggestion and even going as setting their relationship to girlfriend/boyfriend.