Taken from
http://socialsupport.pbworks.com/Shoq%27s-Twitter-Tips#IsYourNameFilteredOutnbspofTwitternbspSearch
(just for backup purpose)
Many users get “filtered out” (they prefer that term, to “banned”) from Twitter’s search index because they unknowlingly violated one ore more of Twitter’s odd Rules that discourage spamming. Twitter does a less than stellar job of making user’s aware of these Rules, and an even worse job of clarifying exactly what the [...]
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Is Your Name “Filtered Out” of Twitter Search