Post Gallery uses internal cache when querying for galleries. Cache is used to optimize performance.
There might be instances, like when migrating servers, that galleries are wrongly referencing media or there are problems with the display. When this happens is recommended to clear cache to force the plugin to regenerate gallery data again.
Clean cache
Cache can be cleaned/cleared by clicking the clean button in the cache settings.
Clean manually
To clean the cache manually, simply delete the folder:
“wp-content/wpmvc/cache”