I know how frustrating that must feel but believe me when I say I take a lot of time checking and double checking that I got every post. That being said there are things that happen on the blockchain that is beyond my control.
I can only vote for posts that I see posted with the openmic tag and even if someone posts everthing correctly sometimes for some inexplicable reason they still do not show up in my feed.
Last week the Steem blockchain was down, but this sometimes happens even when everyting is working normally.
Watch this video of me catching a post disappear before my eyes
When that happens there is nothing I can really do about it.