There is nothing really you can do about this.
As it was mentioned, older episodes are seen by more people and often liked more. Also, people tend to ‘like’ the first episode of a new series more in support of the creator. Later they just read them without necessarily hitting the like button.
Also, not all your active readers immediately check out your newest update. With weekly updates it sometimes takes the whole week or longer until everyone got around reading the newest episode (some even wait for several updates to binge). Hence the fewer likes.
Also, just to let you know, on my series the likes have cut nearly in half since EP1, but my readers are still super active and chatty in the comments. As far as I understand your readers are quite active in the comments too. And that’s what really matters
Therefore I wouldn’t sweat over the likes
That decline is just natural, imo
I think more important is to keep the series rolling and not letting the numbers bring you down.
If it really bothers you a lot, you can always ask your readers to hit the like button. Webtoon readers are usually very eager to help with such a small gesture and when we asked our audience to do that on our other comic, we usually get a spike in likes for a while 