You are not doing anything wrong. Searches using the sites engine often results in failure, more so for some users than others. This is a known issue resulting from the server being used beyond capacity (IIRC). They do want to upgrade the server, but have yet to do so. A quote from the news feed:
Originally Posted by GITP news, 1/7/2012
One thing that is coming along is the dream of a separate server for the forum. While the ad revenue has been rocky, we've also come up with some ideas that may lower the cost (but require additional technical research). Our hope is to meet somewhere in the middle over the next few months, so that the forum can finally have the server that it so sorely needs.
Patience, and the issues will eventually be resolved.
In the mean time, feel free to try using a google search with "site=giantitp.com" as part of the search.