Trying to understand why all my shots were not accepted

The focus is not right on the spot. It can be the major reason for rejection.

7809-3: (colour) noise and generally not a great composition: 

Buyers can make their own cropping, but they can only with great pain add space. You need to stage such assets carefully. It's not (or quite rarely) a good idea to photograph the live mess like this. The DOF is too narrow. The tree may need a property release, even if it is a children's drawing, the creator has the copyright on this.

7550: you have ovesaturated the colours, contrast is missing and your DOF is too narrow.

7587:  The woodpile is not level, the left border is out of focus, and overall contrast is missing.

I did not check the exposure for your assets, but I would guess, that looking at the histogram, I would see issues, especially with the second asset (7550).

The first two images are not in focus.

Hello,

Personally, I think the composition is not that good, esp, in the 'paint' shot - some rather distracting elements, and the flower shot, the composition isn't that good, as well as the overall exposure.  

Secondly, the white balance is off - a bit too yellow. The pile of wood photo - the white balance needs to be corrected.

As you're new, read these links from Adobe help.

User guide:
https://helpx.adobe.com/stock/contributor/user-guide.html
Exposure:
https://www.adobe.com/creativecloud/photography/discover/exposure-in-photography.html
Composition:
https://www.adobe.com/creativecloud/photography/discover/photo-composition.html
Learn and support:
https://helpx.adobe.com/support/stock-contributor.html

As for the wall of wood, not sure what to tell you, so wait for others to chime in. I wouldn't be hesitant myself to submit it, at least as having potential for wallpaper or a graphic resource. I messed with contrast, exposure, adjusting highlights and shadows, etc., but the differences were negligible in terms of quality.

For the flower image, there is a lot of detail lost in the highlights. I won't go into how many flower images are already in the database and how much competition there is...well...I guess I got into it. So there's that. 🙂

The school desk...is that a leaf? It could have been removed.