Reasons for this rejection due to quality issues?

1. Too blurry. The delphiniums are not in focus.
2. Too much shadow and the Boxelder bug is out of focus. 
3. No commercial value for partial/clipped flowers.

For comparison, this is what you’re competing with in Stock:

https://stock.adobe.com/search?k=blue+delphinium

https://stock.adobe.com/search?k=boxelder+bug

https://stock.adobe.com/search?k=orange+flower  6.3 million results

Hope that helps.

I assume you submitted these without your copyright symbol? The first one is out of focus. First and last could be cropped better. In any event, there are over 77 million search results for flowers on Adobe stock. Lot of competition even if accepted.

Crop the whole pictures. Make a main subject