Proposing

• Are both Sims active Sims ? If not, is the inactive one a 'role sim' (consigment store clerk or anything along these lines) ?
• Did either of them have a prior relationship ?
• Are you only using interactions from the romantic menu without interrupting them with any from other menues ?

One of them is an inactive sim, however he is not a role sim.
One of them used to have a romantic interest before, but not anymore.
I am only using romantic actions.

Did they break if off or what was the reason they're not anymore ?

Sometimes the game doesn't recognize properly that relationsships are not valid anymore, depending on what broke it off.

They told them they just wanted to be friends.

That should normally take care of it.

Ii would try if you can ask the boyfriend to move in with you and then check if you can get them to go steady once he's an active Sim.

They are already going steady, I just can't get them to propose marriage

Then move them in and then try to get married 😉