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A screenshot of your Auto-DJ settings might be helpful.
The "different" slider affects how often a particular artist appears, not a particular track. Setting it all the way to different could actually cause tracks by artists that you have fewer tracks by to repeat more often. (Say you have 10 tracks by Artist 1 and 1 track by Artist 2. Because MB is trying not to repeat Artist 1, you might hear each track by Artist 1 once, and the single track by Artist 2 ten times.) You will probably be happier with the results you get from "random" selection.
You can also use a filter rule like "last played ... is not in the last ... [time frame]" to keep tracks from repeating too often.