It is a strange thing.
I have like 10 different accounts with Amazon. All accounts can write product reviews fine but when I vote on a review:
"Was this review helpful? Yes/No"
Only one of my vote counts. It doesn't matter how many times I vote with all my other accounts.
I've tried using different IP address (hide-my-ip), anonym.to and incognito chrome browser.
Amazon seems to know that I am voting twice and they are not counting my vote.