My photos were rejected due to quality issues.

You won't make sales with plants, flowers & trees.  Stock has gazillions of those already.

Capture more unique subjects that include people at work, leisure, play or social gatherings.

As always, the only important information is "Quality Issues". the rest of the stock text may or may not be relevant to your situation. The moderator does not tick anyting of that. 

You used a kind of HDR filtering mode, that creates nice effects, but Adobe will refuse those assets for quality issues. Adobe prefers naturally looking pictures that are correctly exposed (with the histogram going from blacks to the whites, without missing spectrum). A buyer can easily create a look like yours, but they cannot return to a natural look, if they need to. So yes, there is too much filtering.

In addition, composition is an issue. Your first has this out of focus blob quite exactly on the golden ratio lines. (I've drawn the lines approximatively, so don't nail me for a pixel accurate cross line): 

Your second would be better off without that disturbing out of focus element, too, even if it is not exactly on the golden ratio lines: 

I would probably crop your last like this: 

But as the flowers are closed, the picture is anyhow not that interesting. 

Not to misunderstand me: your pictures will have a great success on social media, but on stock you will have to think differently. More craft than art.

Focus is an issue in each of these. The first image has very little in focus and it looks like you've applied a vignette, which is not allowed. 
The second image is poorly composed and focused as well as underexposed.

I can't find anything in focus in the third image, and there's also chroma noise. 

Look at some of the many millions of floral images already in the Adobe Stock database and you'll see the level of quality that is expected. I wouldn't waste much time on capturing flowers for submission to Adobe Stock, as this is a very saturated category, and it's unlikely that a Buyer would ever see or license your image. However, photographing and editing flowers for your enjoyment is a good way practice your skills.

1) Focusing issues or motion blur on the foreground leaves

2) Underexposed

3) Again, focusing issues

There are 27,639,547 images of flowers on Adobe Stock. That is your competition. Keep that in mind when it comes to submitting flowers.