<p>I have only a little to add to @NickCox's answer.</p>

<p>Down-votes do tend to be sticky, which is a pity when the down-voted poster makes the effort to improve their post. I don't see any reason to suppose this is due to anything more than people's disinclination to keep on returning to a post they've down-voted to see whether it's been improved. There are some feature requests on Meta SE that you might be interested in:</p>

<ul>
<li><p><a href="https://meta.stackexchange.com/q/1751/225179">Allow an edit to notify downvoters: “I think I've fixed the issue
now - please check”</a></p></li>
<li><p><a href="https://meta.stackexchange.com/q/165962/225179">Allow me to get notifications on specific down-voted
questions</a></p></li>
<li><p><a href="https://meta.stackexchange.com/q/247608/225179">Option to be notified when a post I downvoted is
edited</a></p></li>
</ul>

<p>I'd like to see something of the sort implemented. On the other hand I have the impression that for "controversial" posts with several down-votes &amp; several up-votes it's often the case that the up-votes came after an improvement, &amp; after the down-votes&mdash;that people are trying to rectify a now-undeservedly low score.</p>
