I used Picard to tag all my MP3 files, but now, a couple of months later, if I reload the files in Picard some tracks no longer show the checkmark in the right pane but have reverted to the green box icon. Why is this?
Hi!
That is probably because something has changed in the MusicBrainz data (for example, someone might have added some extra info or whatnot).
I think it can also happen if there’s some data that just can’t be saved into your file format (for example, if it only stores year but we have year-month-day) but I might be misremembering.
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Updates to the MusicBrainz database sound like a very probable cause. Thanks for the suggestion.
Updates are quick to spot. Click on a track and you’ll see tag differences flagged in yellow.
Not all differences are actual differences.
For example you may have a different length recording but all other data tags the same. This flags as “different”.
Sometimes the updates can be really hard to spot. A classic one that catches me out are the hyphens and dashes. There are often edits of the data just to swap a dash to a hyphen in a title. I can never see these differences by eye - but they happen.
Similar there are corrections to Capitalization to make the data more consistent.
Also add in people expanding the data stored. Adding ASIN numbers and other links. Adding more cover art scans. Adding details of who wrote the track and who played on trumpet and so forth.
Much of the time you may find the “changes” in your MP3s are not actually enough of a change to need to update your tags. Have a glance through your tracks looking for those yellow differences. You’ll soon get the hang of how many of the changes are relevant to you or not. It is then quick to hit that SAVE button to update that set of tags. 
(I would certainly avoid trying to update ALL tracks at the same time as that can bog down Picard if it is updating hundreds of files at a time. Just do one album at a time to keep it sane. 
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