If you see that happening frequently, it's just a coincidence. It isn't you. There are various factors that alter the payout of a post:

1. The ever changing value of Steem. If the value of Steem decreases, but SBD doesn't, then 1 SBD is suddenly worth more Steem than it was a moment ago, so the amount of SBD listed for payout will be less.
2. There is a fixed amount of daily payout on steem. Let's say, for the sake of illustration it's $10,000. If I'm the only person to vote today and I vote on your article, then your payout would be $10,000. But if someone else with equal voting power comes along and votes on another article, your payout is cut in half to $5,000. If you see the payment on a post decrease, it could be because a lot of people (or a lot of voting power) is going toward other posts and increasing their payout.

Don't worry. Your upvote will always increase the percentage of the daily payout that that post will achieve, even if the actual numbers go down.