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Not sure you just go to the bottom of https://steemit.com/~witnesses
And type in my username and hit vote

This was what i did.
Screenshot_20180517-111134.png

Maybe unvote and revote for me? I'm really confused as to why it doesn't show up for me

Just checked your account and you maxed your votes. That's why.

Also roadscape and steempty aren't active witnesses anymore. So feel free to unvote them and vote for me instead :P