It could also mean that you've made too many votes and your voting power has reduced to 0, this is especially true for those who's upvote may usually only be worth $0.01.
If their vote power becomes less than 100, it's unlikely to be shown on the payout indicator when they upvote.
Just thought I'd add a further possible explanation to the vote not adding any reward.
RE: Why my upvotes won't give credits for sometimes?