I'd like to add that one important factor in slowburn is friendship.
Like others already said, people who only slowly develop romantic/sexual feelings for each other (one isn't the other, keep that in mind, as well!) might not even be aware of it. My partner and I are actually a good example of that, as we had been seeing each other for months as "friends" and "you're like a brother/sister to me" before we, after going on a concert together, realized that, hey wait, there's another layer to our relationship. While pretty much all our friends were just like: "FINALLY".
So, other people might see what's going on between your characters before they know it themselves. Maybe a friend of one of them teases about it. Maybe there are insinuations from their friend group that just go completely over their heads. The fun in slowburn, I would say, lies in the reader (and other characters) exactly knowing what is going on while at least one of the two people is oblivious to it. Maybe the other even pines. Maybe both start to pine for the other after a while, but none of them actually thinks that they'd have a chance. Or they don't want to ruin their friendship.
Make your readers suffer.