I have seen a lot of people have mixed results with starters. Generally the problem is that they make too small a starter. Most instructions say to use some dme in 2 cups of water. This is too small to get any yeast growth. You might as well use no starter at all and you will get the same results.
The minimum amount of starter wort to use to get a sufficient amount of yeast is about 2 quarts. Combine this with proper aeration and you will consistently get activity in less than 5-6 hours.
To figure out just how big a starter you need, check out the pitching calculator at www.mrmalty.com
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