So, you know you have these two characters, and you want them to slowly fall in love with each other -- what are the obstacles to that?
And I don't mean like, "think of a trope that could get in their way!" but that like, any time you put two humans together, there will be obstacles in interacting with someone who isn't you and doesn't think like you. What are those SPECIFIC differences for these two people? Like if one person is shy, and the other person is outgoing, and so the shy person avoids this outgoing person because they come on so strong -- what happens to finally push both of them to realise and confront this difference and figure out how they're going to compromise?
Something I did for some of my characters in a non-romance story that helped me a lot was ask the question "what do they have to learn" and "who or what is going to teach them?" So like, if you have a character who's been burned before that has to learn to trust again, what's something that's going to challenge their idea that you have to avoid trusting others? If you have a character who never stands up for themselves, what's going to teach them they CAN stand up for themselves, and what -- positive or negative -- is going to push them to finally DO it?
You know where you want your characters to be at the end of this story -- what experiences do they need to have to bring them to that? Because hopefully, the person they are at the end is different in someway - you don't ever meet another person who already fits you perfectly; you have to compromise and learn about both yourself and the other person to figure out how to be partners.
And on the other hand, what experiences are going to show each of them what's beautiful about the other person? Like if you have a character who acts cold and aloof, but goes out of his way to look out for and protect vulnerable people, then something needs to happen to let his love interest see that side of him. If another character seems naive and silly, but is actually a kind but determined person who won't let anyone push him aside or do harm, then his love interest may come to respect his kindness when they see the strength behind it -- what's going to happen to make his love interest see him in a new way?
Etcetera etcetera -- obviously these are all just examples that probably don't fit your specific characters, but it might be a direction to help with thinking about Scenes and Choices and Challenges that your characters might face as they figure out their feelings for each other!!