Thomas, It’s all about contrast. Lights vs darks. Your colors are too analogous, too close together on the color wheel. To create definition and form, you need to place darker colors next to the lights. Don’t mix them together, let one layer dry before putting on the next. I did a quick photoshop version to demonstrate the importance of value change. I hop e this helps.

Rich

I think you need to be more patient , let a layer dry before adding another one !

The comments are very valuable. I would like to know what paper you are using as well as what brands of paint?

this might help ..”A” and ‘B’,and “C’ are the same color,,,it depends on the surroundings

I have one more suggestion to add to the excellent ones already given….
leave some white paper. Keep at it! Things will get easier.

Well, I like the first sunflower, and that it’s not bright and contrasty. It has character.

What I think you might do is put some activity in the pollen and petals—not too much, enough to energize the tones (make less flat.)

I think the backgrounds are nice, and set a moodiness.

For future work: think about using complement colors for shadows, while doing the underpainting. So in this case, violet under the yellow.