Hi, I just read your post, and the comments. I'm surprised that nobody told you the correct answer, yet! Here it is:

Whenever a post or comment on Steemit gets an upvote, some STEEM (and SBD) is added to the potential payout (after 7 days). Whales, of course, add lots of STEEM and SBD, but most Steemians aren't whales, and their upvotes add only a little STEEM and SBD. Then, Steemit (or whatever frontend you are using) uses the current USD price of STEEM and SBD to update the potential payout in US Dollars.

Imagine you wrote a post 3 days ago and it got about $20 in upvotes from your followers in the first 24 hours. Imagine the USD price of Steem falls from $0.5 to $0.4 today. If your post doesn't get any more votes, it will still say $20 potential payout. But if it receives a new vote, even if that doesn't change the potential STEEM and SBD payout much, it will appear to drop down to about $16.

That's why you believe you are losing payouts when new upvotes happen. The price of STEEM has been volatile (up and down) lately. When a new vote came to your old post, Steemit added their STEEM and SBD, and updated the post to show the new USD value of your potential payout. Because the USD price of STEEM was down, the USD value of the post fell. This is completely normal, and has been happening all along - you just haven't noticed it before.

Here's another example. Sometimes, I find a post that is a few days old, and when I upvote it, instead of jumping up by $0.04 (the value of my upvote), it goes up by several dollars (for example, from $41 to $47)! This makes me feel very powerful and rich, of course... haha! But in reality, the reason it jumps up so much with my upvote, is only that the price of STEEM had risen since the previous vote.