you say the two possible reasons you are not making the front page are: your content is weak, or people's taste's have changed. The fact that the number of submissions has not changed suggest to me a third and more plausible option: The quality of submissions, and therefore the competition for the "front page" has increased.
No. You can have a domain of discourse where ALL content is weak. In fact, you can have content in this domain that is stronger than most of the content, but it might still be weak.
