I've had nothing but issues with the search function on forums. One time in particular, on a different site, I knew the title of a thread with proper capitalization and it refused to show no matter how broad or specific I made the extra search criteria. I've found the only useful way for me to search a forum is to pick a search engine online and type in site:permies.com (or whatever site you want to search) followed by what I am searching.
I've noticed here that if you go into a subforum and click the search function in the top bar that it defaults to searching that subforum. I'm not certain if that is by design or not.
If you know the specific user who posted something then you can click on their name to get to their profile and search their past posts or threads they have started.
In any event, here is an example of a quick search I did for -
buckwheat.