I like the colors! They are very bright and saturated but not too much, the toned down simple background helps a lot, if it was any brighter/had more contrast/was more complex it would've been too overwhelming.
If you are not happy with it perhaps it's a question of texture? I took a look through your gallery and you tend to rely on, what I assume to be, the airbrush tool, which makes the shading rather blurry. Some like it, some think it makes the pieces too artificial looking.
With the tree in particular there seems to be a problem with rendering leaves. The brush strokes are a bit too big, look up tutorials on how to render trees in digital art.
"LEARN ONCE FOR ALL HOW TO PAINT TREES (Photoshop painting process+ Voice)" by GabyT on YouTube might suit your style from what I saw by skimming through it https://youtu.be/r2EF5ieMt4s
Focus on texture. In some places it will be soft in some it needs to be crisp. I recommend