Three main factors determine if your fan-page's posts are displayed to your fans.
How interesting is your content? If you post every day and nobody interacts, Facebook assumes you are not interesting - you lose. That also happens if you buy fans, of course. They don't interact at all - or at least not naturally.
What type of content have you posted? An image will generally work best (without a link in the description). Videos are OK, simple text must be very short to work. Keep in mind: the post is displayed next to lots of other content - if people can't understand in a glance, they won't interact.
Your last week's post won't work today anymore. In news streams, things disappear gradually. A really engaging post will stay higher up in the stream for a bit longer - but you have to keep posting relevant, helpful content to stay visible.