I'm going to go out on a limb for this one and say you haven't been drawing very long.
Like not very long at all.
Like less than 6 months.
If that isn't the case, then you haven't been improving at an appreciable rate, and that's a problem.
Now I'm not here to discourage anyone or tell them that they aren't good enough or need to give up. That's not the point of this thread. My purpose here is to be blunt and honest so that you can actually get better at what you do.
In your case, it's all about the basics.
Let's take a look at your comic's depiction of a castle on a forested hill:
Now let's take a look at an actual castle on a forested hill:
do we see the difference?
Now obviously I'm not going to tell you that you need to be going for perfect photorealism. Very, VERY few artists can manage that in the first place, and I can't think of a single one who can manage it fast enough to make a comic out of it.
However, pay attention to all the things you don't think you need to pay attention to:
On the left here, I drew a very cartoonish clip-art-y representation of a pine tree, and you can recognize that it's technically supposed to be a pine tree because christmas ornaments and car air fresheners exist, and it generally follows the understanding you have in your mind of what a pine tree looks like.
On the right, I have a very rough and simplified sketch of what I drew when I went on google and looked up 'pine tree' and drew from that reference. It's far, far more complicated, The branches actually lean up a little bit, they're not separated as perfectly into layers, they bunch and overlap and point in different directions.
If I were to take this even farther there would be massive pools of light and shadow where the overarching shape that the needles have grown into leave large shapes jutting out or receding in, there would be changing textures all over the place as some areas are dense pine needles and others are where the bark and branches are visible.
My point here is this: Never, ever ever under any circumstances should you just assume that you know how to draw something from memory. Always work from reference. Always go look up how something would look in real life.
Even if you aren't drawing in a realistic style, you should know how to draw something that way. Every single art style, no matter how cartoony or exaggerated, is derived from reality: it is based on, in some way shape or form, the things you and others see around you. If you don't know how to represent something as it actually looks in the real world, then you can't know how you're distorting or exaggerating it for your art style.
Practice those fundamentals, even if you think you don't need to. Learn how to draw things looking like they take up real, solid, three-dimensional space, learn how to indicate texture, either through your lineart or through your colors, learn how to represent things as they actually look, and then you can start to stylize.