@rsew6-8 The items from the marriage collection are only necessary to complete the wedding quest. Once the quest is finished, you won't need the arch, lattice or pillars to make your Sims engage and marry.
Could it be that the other Sim has retired? Retired Sims can't marry.
Is it one of your own Sims, a townie or a Sim that belongs to another player?
I would try doing another hangout between these two Sims and see if the Proposal event appears. Chapter events don't have the proposal option.
It's not possible to start a new relationship story between the same couple of Sims, so you would have to try with a different one.
@rsew6-8 Townies or Non Playable Characters (NPCs) don't have a plumbob over their heads.
Sims with a blue plumbob over their heads belong to another player.
Retired Sims have their backs a bit hunched and get grayish hair.
There is no breakup option on regular stories. The story will not disappear from your Sim's tab. You just need to start a story with another Sim.
If your Sim marries an NPC from town, it will immediately move in to your household and then you will be able to promote them to playable if you want to.
