Who are you talking about? All three of them had at least one kiss scene where their lips touched. If you're talking about Preuk and Sin, the camera didn't show their kiss in the 1st ep, but they had actual kiss scenes in the subsequent eps.
Idk why people here are making it seems like she is flopping now lol. She’s doing fine, though I do agree she could be picking better projects to reach her full potentials. Her name is still well-known in SK. Even Kim So Hyun’s last 2 dramas (in public broadcast channels) didn’t reach double digit. Kim Yoo Jung hasn’t had much projects in the last few years (keep in mind she also had hypothyroidism) so it makes sense that her popularity isn’t the same. Backstreet Rookie had a lot of controversies but in terms of ratings, it wasn’t that bad. She’s also coming back to the big screen so hopefully they turn out well.