I'm sorry I don't have a rec, but I wonder if you're portraying the 'slow' part of it and not the 'burn'? I am not someone who can write slow burn either, but I think I do alright with UST, so if it needs to go a touch 'burnier', maybe focus on physical stuff, noticing little details about the other person, perhaps experiencing physical stuff when they're around - heightened sensation, prickling skin etc. Nothing MAJOR but small things. Noticing their cologne, little tells like holding their gaze a tiny bit too long, thinking about them when they're not there, perhaps finding themselves absentmindedly touching an object belonging to the other person (eg stroking the wool of a coat hanging on a peg).

Idk. What is slow burn anyway? Is it like, attraction but it takes them forever to get together? Or is it that they are friends for a large part of the fic and aren't attracted to each other until later? I'm sorry, I'm not much use at all!