I asked Doc the same question! Here is the reply he sent me:
The best time to try "search" is between midnight and 6:00am when there are few on the site.
Search is always turned on, but disables itself when the load on the site is too great. If "search" were enabled all the time it would take 5-10 minutes to download threads on the boards- thus the limits on "search". We often have 1100+ posters on the boards at one time - which is quite a load for any system. We hope to alleviate some of that when we get new servers- they have already been ordered.
Stay tuned!
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Doc
Assistant Webmaster DIS