Retrieving posts from Instagram can sometimes take a while, which can slow down your site. That's why Spotlight saves all imported Instagram posts in a cache, which is a fancy word for temporary storage.
When Spotlight gets posts from Instagram they will be saved in a cache for a limited amount of time. Within that time, Spotlight will get posts from the cache rather than from Instagram, which makes things much faster. However, this also means that during that time Spotlight will not be able to fetch new posts from Instagram.
How long does the cache last?
By default, posts are kept in the cache for 1 hour. You can change this from the Settings > Configuration page using the Check for new posts option.
Clearing the cache
If you are having problems with some of your posts it would be inconvenient to have to wait for the cache to expire for those posts to be retrieved again from Instagram. So, if you need to force Spotlight to re-fetch the posts from Instagram, you can manually clear the cache from Settings > Tools > Cache > Clear the cache.
Clearing the cache is a harmless operation. This will simply remove all cached posts which forces Spotlight to get them from Instagram once more. Note that the fetched posts will be saved in the cache again once they are re-fetched.