Two possibilities.
The first is that you trying fewer crosses than what you calculated for. Check that you're not going for the wrong hole in the rim.
The second possibility is that you have your flanges phased wrong. Sight straight across the hub looking at the near pairs of spokes going in the same direction. If the one in the left flange is to the left in the hub, it must be to the left in the rim. This kind of error is what causes the long-short-long-short kind of problem people often encounter.
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