We can only use our experience to guess. Other people with more skills than I may see different problems, but I observe the composition could, in my opinion, be improved. People may want to do their own cropping and may want to see more of the dog, but the most serious composition issue is that you do not have the very top of the head in the shot. The one absolute stopper is you've signed the picture. Did you ever see an advert with the photographer's signature...?
At first I thought the purple tongue was a color balance issue but I now realize that's a typical Chow trait.
I think TSN nailed the #1 rejection reason: signature watermark. Try removing signature and resubmit.
There are also completely blown out areas on the right and left side of the dogs head.
Apart from the signature, I see you've tried to recover the highlights on the dog's fur, but in doing so, you've ended up with a grey dull area!
Exposure could be improved.
Also, I think you've too tightly framed the dog, so I agree with TSN about the composition. Give the dog more room to move.