Here are a few suggestions that will help you for future submissions.
Image Caption / Image Title
The correct description plays a huge role in the image being accepted or not.
An image such as this, needs a caption such as the following, or something to this effect
“A single in focus strand of purple lavender against a sea of blurry and out of focus purple flowers”
Because the depth of field in your image is so shallow, a caption such as this ensures that the image is reviewed correctly.
When taking images handheld, most images suffer from Motion blur.
Images such as flowers in a field, suffer additionally from wind!
So, even if you were using a tripod, there is no guarantee that the shutter speed was sufficient enough to capture a pin sharp image.
In the case of this image, the main subject it is not sharp and suffers from motion blur of about -70-75 degrees(going upwards towards 11oclock) at a radius of about 8-10 pixels.
This needs correcting.
In addition to this, the main subject also lacks contrast and definition and that also increases the possibility of the main subject being viewed as out of focus.
The motion blur around the main subject along with the back/side light, has created a Blue/Magenta tint that needs to be taken out.
Finally, your camera plays a big part on the quality of the image.
What may look good on the back of the camera or great on a monitor, may not transpire to look so great when its viewed at 200%.
You have to remember, it is the Stock agencies reputation at stake and therefore your image will be scrutinised at not just a maximum of 100% resolution, but possibly a lot higher.
Along with all these technical issues, your image also needs to highlight the main subject so that it’s the brightest part of the image and where your eyes settles first.
Other brighter areas need to have the highlights reduced.
Perhaps, for this type of image, you need a lens that can provide a more “creamier” background blur so that there is a clear definition between the subject and background with the right amount of contrast.
Due to the image being backlit, or slightly to the side, I can only presume you have lifted the shadows or the camera automation has lifted the shadows and this has introduced noise.
Hope this helps