On the rating aspect... there seem to be a bunch of initial haters on webtoons, who seem to downvote new series a lot... i had that too when i started there a year ago. There votes get less noticeable the more normal votes you get though.
My rule of thumb on the webtoonsrating would be to just ignore it anyway... a rating doesn't show the true quality of a series.
If your subs don't or just slowly increase this can have multiple reasons... first if you don't upload frequently (i beleve once every two weeks or so, but don't quote me on that) you get ranked down in the ,popular section' a lot... which will result in a decrease of subs gained... second your series isn't ,new' anymore. I feel like series which aren't on webtoons for long get a little boost in their system to attract an initial audience...
Your uploadtime? I don't have much insight in this topic but there seem to be times where significan more people are online and browsing through fresh that othertimes...
And well the obvious factors to gain an audience: coverartwork, description, etc. i won't go into detail on this....
Hope i was able to help somehow.