An undocumented (as far as I know) feature of stackexchange.com tag filters is that it doesn't show closed questions. This makes some sense if you see them a a way to follow good questions across multiple sites, though it is somewhat confusing that the tag filters work differently than the newest questions tab on the sites themselves.
There's an additional feature that hides downvoted questions which I discovered when I wondered why I wasn't seeing certain questions in my tag filter. I thought initially it was triggered by the same threshold that hides questions on the frontpage (below -4 for most sites), but that isn't true. All questions with a score of -1 and below are hidden, which doesn't make any sense to me, so I'm not sure if this is a bug or a feature.
In any case I think this should be changed, and downvoted questions should appear in tag filters.