Well, to be honest as someone who came to Webtoon a bit later than you, this is pretty average and normal for a series which romance isn't its main genre.
Right now Webtoon has such a big amount of creators from all over the world, a lot of people making several series besides that now peeople don't bother engaging much with subscribing, commenting or giving a like, and it has been a common phenomenon since a few years ago. Right now is the "Be satisfied with the view" era if I have to be honest.
Not to mention that changes in algorithm + Webtoon favouring creators that already don't even require their support to get noticed or be considered a hidden gem, its become harder to gain readers. And let's not forget that there are even more websites / apps where people can read, so there are less readers overall in a single place compared to artists.
You can always try to share on all your social media whenever there is an update, tiktok, youtube and even twitch could be used to share the process or sneak-peeks of episodes. And to post your comic on multiple platforms and be satisfied with the sum of engagement from each place.