I have performed some more testing on this issue on a fresh SL4 project (no styling of my own)...

I set up a timeline with 2 datetime series and an entry in each series like so:

            this.xamTimeline1.Axis = new DateTimeAxis()
            {
                AutoRange = false,
                Unit = 2,
                UnitType = DateTimeUnitType.Hours,
                ShowLabels = true,
                ShowMajorGridLines = true,
                ShowMajorTickMarks = true,
                ShowMinorGridLines = true,
                ShowMinorTickMarks = true,
                ShowThumb = true               
            };
            (this.xamTimeline1.Axis as DateTimeAxis).Minimum = new DateTime(2010, 7, 6);
            (this.xamTimeline1.Axis as DateTimeAxis).Maximum = new DateTime(2010, 7, 7).AddSeconds(-1);

            DateTimeSeries one = new DateTimeSeries();
            DateTimeSeries two = new DateTimeSeries();
            //DateTimeSeries three = new DateTimeSeries();
            //DateTimeSeries four = new DateTimeSeries();

            DateTimeEntry dte = new DateTimeEntry();
            dte.Time = new DateTime(2010,7,6,10,48,29);
            dte.Duration = new TimeSpan(0,0,59);
            one.Fill = new SolidColorBrush(Color.FromArgb(50,0,0,255));
            one.Entries.Add(dte);

            DateTimeEntry dte2 = new DateTimeEntry();
            dte2.Time = new DateTime(2010, 7, 6, 10, 49, 01);
            dte2.Duration = new TimeSpan(0, 0, 21);
            var ctl = new EventSpan();
           

           
            two.Fill = new SolidColorBrush(Color.FromArgb(50, 255, 0, 0));
            two.Entries.Add(dte2);

            this.xamTimeline1.Series.Add(one);
            this.xamTimeline1.Series.Add(two);

When I run the app, zoom right in with the zoombar and widen my browser window, I can see the eventspans rendering as expected:

However, If I shrink the width of the browser window, by just a few pixels here, I lose the entry from series one....

You see that the eventspan has vanished, and the ability to select it with the thumb has also gone.

Once again, thanks for any help here...