Complaining doesn't help your business
You cannot improve your income by complaining because Shutterstock has rejected one of your images.
I'm talking about a single rejection, because if instead, microstocks never select your photos, then you will need to take action to understand what's wrong with your production. There is also the chance that you are not a good photographer.
I know that in a world where someone says that everybody should have the right to do the work they love it sounds bad to know that you may not be someone who can make a living by selling his own images, but it's much better to understand such things now instead of wasting months to create something that you will never sell.
This said, if Shutterstock or another agency rejects your photo, and you really want to have that photo published, the best strategy is to upload it again, as if nothing happened. You can do it because agencies don't have software which recognizes double uploads. If you find a different reviewer the second time, your photo will have a chance to be accepted. If knowing this makes you feel better, then do it, even if it doesn’t change your total income.
I repeat: this is not a good strategy, and it's even worse if you have the assumption that the reviewer is wrong because another agency has accepted that same photo.
Adobe Stock selected my stock image, so Shutterstock should do the same.
Agencies don't care what their competitors do on single contents. Every microstock has its own policy, and all of them have more than 100 million photos online, so they don't care about the single stock image—mine, yours or every other producer's.
Selling photos and videos is a paid job, so it's not art; it's a business. The whole world, whether you like it or not, lives to make money. In microstock, successful producers who make thousands of dollars every month can do so only because they work like professionals, and complaining about a single rejection is not compatible with a professional behaviour.