adobe rejects the image due to poor technical quality, but I don't know what they mean.

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The eye is automatically drawn to the brightest area in the image which is the sky, so it competes with the fern. Most of the edges edges of the fern are blurred, which looks like the "fake blur" effect when the iPhine Portrait mode is used. 

The fern needs some sharpening. There appears to be some movement around some of the ferns edges. The shadows need to be lightened some. The green blob on the right needs to be removed. I personally, would add some clouds so that the highlighted area is not some bright. 

I am trying to get a grip on this, also. From my rejections, adobe does not seem to like a very narrow depth of field. The edges of the fern leaf looks a little hazy also.  Proof would be to take the same shot with a greater depth of field may be to the other extreme. I talked to one pro shooter that now gets less than 1% rejection so it is possible