My sims keep rejecting each other when it comes to romance.

There are many things that can influence the success/failure of romantic interactions. While your sims are chatting, check the box at the top of the screen where it says level/type of conversation (ex: amorous exchange, suggestive encounter, etc.). If it's a negative one, the romantic interactions probably won't work. Make sure neither sim is embarrassed, sad, etc., and check if there are any conflicting traits. If your sim does a failed romantic interaction and gets an embarrassed buff, I would probably wait a bit before trying anything again.

What I usually do is start off with friendly interactions, and then some of the basic romantic ones like pick up lines and compliments. After those interactions are successful and the conversation becomes more flirty, then I move on to the risker ones like kisses.