Thread:Project:Support desk/Search doesn't find all articles

I use MediaWiki 1.18.1, PHP 5.3.3 and PostgreSQL 8.4.9 (UTF8 and german locale). Search works well, but some articles aren't found, for instance: I search "urlaub" (german for holiday) and I get some articles containing this word. But I don't get an article titled "Urlaubsvertretung" which contains this word too. If I search for some other words, that are part of this article, then I find the article titled "Urlaubsvertretung". This behaviour occurs only while searching some "special" words. Generally searching works fine and shows the correct result. Whats the reason for this behaviour and how to change it?

Thx