@ThatDudeBlue Is this happening with other couples with 'normal' relationship-type traits and likes/dislikes, etc.... it other words, Sims for whom no traits indicate that either of them would be more challenging to get into a relationship, such as unflirty, socially awkward, etc.
I have done things like move them both into the same household/home so I could control both of their reactions, doing events or actions that are oozing with romance, and so forth. You could also try cheating their needs to not decay and have them take a small vacation from work so they can have time to not worry about other things. Can you include a screenshot of their traits, relationship levels, and mood?