In Magento, whenever you make changes to products, static blocks, etc, it recognizes that the data in the database is no longer the same as what it has in the cache. Unfortunately, Magento doesn’t realize what cache data is different, just that something is different. There are many reasons a block cache is invalidated from updates to products, catalog price rule changes, and 3rd party extensions. Also running of cron jobs can cause block caches to become invalidated as well. You will need to go into System > Cache Management and refresh the invalidated cache types.