Pictures rejected, i can't see any valid reason.

Use the Histogram panel in Photoshop or Lightroom to check your exposure levels.

Adjust Curves or Levels accordingly. 

I opened up the shadows on this one. Maybe a bit too much, but you can see there's a lot of missing detail that's worth seeing.

Personally, I would clean up some of the distractions from the mushrooms, like the fly and sticks on the red one and the large dirt splotch on top of the white one. Edge sharpness may also be an issue for the editors on the mushrooms. They seem to be getting more picky on minor details these days. So maybe a clean up, slight sharpen, resubmit

These are pretty good, perhaps a bit underexposed in the shadows and too tightly cropped.  You can consider editing slightly and resubmit for consideration.