The most common reason why a syndicated account will not update is that there is some sort of problem with the RSS feed that it is syndicating from.
-- The site must be a valid RSS feed. A normal HTML page will not work.
-- The feed must contain valid RSS. You can check that the feed's code is valid by using an RSS validator. One can usually be found by performing a query for "RSS validator" on your favorite search engine.
If no updates are appearing, it is also possible that the feed itself is not updating, even if it contains valid RSS and even if the site that it is syndicating from is updating. If you are sure that the site is updating, then you should contact the maintainer of the website being syndicated to correct any invalid RSS feed problems.