Make sure you are at Bitcoin Forum not a subforum or thread.
For example look at the upper left.
Notice it says:
Bitcoin Forum > Other > Newbies (Moderators: psy, SomeoneWeird, MiningBuddy, tysat) > Searching this Forum - doesn't seem to work
If you search right now the search is limited to this thread.
If you click on the link to the Newbies forum and search the search is limited to just the Newbies forum.
If you click on the link to "Bitcoin Forum" and search the search covers the entire forum.
Make sure you are searching in the right place. Also if you have any sub-forums hidden, posts in those forums won't be included in search results.