I see what you mean
The forum search has never been 100% though, I’ve found it’s better to use google with: Your search tearm + modx
google "site:modxcms.com/forums/ search terms" will also do a good job.
This is a pain to set up, but you only have to do it once, and once it’s done, forum searches will be a breeze: http://modxcms.com/forums/index.php/topic,30286.msg183962.html#msg183962
And if you don’t use firefox; bookmark this
(then just type in your search words in the end of it)