Why does aggregator report "There is new syndicated content" but still 0 items for this feed?
I have just been experimenting with the aggregator and come across the same issue. The only workaround that I have been able to find in order to pull content from the RSS feed in issue has been to use the Feeds module. If you are still having this issue then this is a possible solution.

EDIT: Just found a far simpler solution for the problem. By default the aggregator deletes items older than 3 months and 3 weeks from the feed. If the youtube feed you are subscribing to has not had new content added in that time then you may get the situation you describe. If that is not the case then go with the first option I suggested. Otherwise you can change how long Drupal keeps the feed items in the Aggregator settings tab.