Find out who's linking you in delicious.com
Because otherwise the plug-in would end up doing too many requests in too little time and delicious.com won't like it -- and will ban you while they are on it.
The default settings correspond to the absolute minimum they suggest. In fact, if you have to change them, it's got to be in order to increase the values, so that the plug-in queries less often.
OK, let's get technical. When somebody visits any post or page in your blog (and I am talking about the WordPress concept of 'page' here), the plug-in checks if we have any information about this page in delicious. If that's the case, and the data hasn't expired yet, there's nothing else to be done. Otherwise, we'll try to query delicious, but only if a minimum amount of time has elapsed since the last time we contacted delicious.
So for this plug-in to work, you need to have visitors. Although if your blog is indexed by search engines, it should be fine because they act as visitors anyway.
Because delicious doesn't provide that data. They only show up to the latest 100 bookmarks. If you want to see more, you have to visit the bookmark page in delicious :)