Are you using CategoryDateTimeAxis?
If yes, then all series must have the same number of points as this axis type measures by index, not date value.
You can ensure series have the same number of points by inserting null or empty points with X=value and Y=double.NaN
Please see our example here which shows how to do this:
The example uses the CategoryDateTimeAxis which collapses overnight and weekend gaps in data, but, uses the Index to the data to calculate X-position. This means that all DataSeries must have the same number of data-points (you can pad DataSeries using double.NaN for null or empty points).