Have you upgraded your page to the new layout? Facebook have made changes to pages and now they order posts by "relevance". How they work out relevance I do not know. A lot of people are complaining at the moment saying they need to bring back chronological order.
If you have the new page layout you can select "admin view" under the "wall" tab and in that you can order by date. Unfortunately it doesn't appear to users. I'm not sure how this affects the old style page layouts