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"Poor Lighting--Poor or uneven lighting, or shadows. White balance may be incorrect."

Is this because I am using CPL filter for my outdoor shots?

Most shots that I submit to SS that have any noticeable shadows are rejected, even if they are the point of the shot. 

I don't even attempt to use CPL's anymore because of how much they increase the noise.  I generally can get the skies blue enough without them, and if I want more I can drop the cyan and blue lightness and increase the saturation to have essentially the same effect as a CPL without the noise.  The only time that I use CPL's for stock is in the studio to control reflected light on non-metal objects.

It's the shadows in your photos. I'll add this. If you use an on camera flash, for something other than fill flash, they will probably be rejected as snapshots or poor lighting.

Anything with heavy shadows, the subject in the shadows, half light and shadows (even if intentional) will get rejected in most cases for lighting.

Put the Sun over your shoulder, instead of heavily from the side, and you're on the way.

Thank you guys. It seems like I need to carry portable reflector for photographing small object (it would probably scare off a squirrel :-) ) and for architecture shots avoid shooting when sun is high ;-)