As scamero said, first mark the tip area, and make sure you're covering the tip.
If you are, and the grind marks are actually changing direction, check this. If you're following the standard directions of moving the stone up and out, you might be moving the stone basically out (the grind marks will be nearly parallel to the edge), so that, as you reach the tip, the stone will actually start moving back down. (Usually discovered when you use a leather strop and it gets cut as you reach the tip). So you may try just slowing down and pay a bit more attention to how the stone is moving as you go down the edge. You can also do tip to heel strokes to insure you're adequately sharpening the tip.